[0/3] Export machine type deprecation info through QMP
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Message ID 20190423212246.3542-1-ehabkost@redhat.com
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Eduardo Habkost April 23, 2019, 9:22 p.m. UTC
This series adds machine type deprecation information to the
output of the `query-machines` QMP command.  With this, libvirt
and management software will be able to show this information to
users and/or suggest changes to VM configuration to avoid
deprecated machine types.

Eduardo Habkost (3):
  qapi: SupportStatusInfo struct
  machine: Use SupportStatusInfo for deprecation info
  qmp: Add deprecation information to query-machines

 qapi/common.json                   | 24 ++++++++++++++++++++++++
 qapi/misc.json                     |  5 ++++-
 include/hw/boards.h                |  7 ++++---
 hw/i386/pc_piix.c                  |  4 +++-
 hw/ppc/prep.c                      |  4 +++-
 vl.c                               | 19 +++++++++++++++----
 tests/acceptance/query_machines.py | 27 +++++++++++++++++++++++++++
 7 files changed, 80 insertions(+), 10 deletions(-)
 create mode 100644 tests/acceptance/query_machines.py

Comments

no-reply@patchew.org April 23, 2019, 9:28 p.m. UTC | #1
Patchew URL: https://patchew.org/QEMU/20190423212246.3542-1-ehabkost@redhat.com/



Hi,

This series seems to have some coding style problems. See output below for
more information:

Type: series
Message-id: 20190423212246.3542-1-ehabkost@redhat.com
Subject: [Qemu-devel] [PATCH 0/3] Export machine type deprecation info through QMP

=== TEST SCRIPT BEGIN ===
#!/bin/bash
git rev-parse base > /dev/null || exit 0
git config --local diff.renamelimit 0
git config --local diff.renames True
git config --local diff.algorithm histogram
./scripts/checkpatch.pl --mailback base..
=== TEST SCRIPT END ===

Updating 3c8cf5a9c21ff8782164d1def7f44bd888713384
From https://github.com/patchew-project/qemu
 t [tag update]            patchew/20190416125744.27770-1-peter.maydell@linaro.org -> patchew/20190416125744.27770-1-peter.maydell@linaro.org
 t [tag update]            patchew/20190418113110.160664-1-borntraeger@de.ibm.com -> patchew/20190418113110.160664-1-borntraeger@de.ibm.com
 t [tag update]            patchew/20190420073442.7488-1-richard.henderson@linaro.org -> patchew/20190420073442.7488-1-richard.henderson@linaro.org
 t [tag update]            patchew/20190422195020.1494-1-philmd@redhat.com -> patchew/20190422195020.1494-1-philmd@redhat.com
 t [tag update]            patchew/20190422221714.7630-1-jsnow@redhat.com -> patchew/20190422221714.7630-1-jsnow@redhat.com
 t [tag update]            patchew/20190423160608.7519-1-thuth@redhat.com -> patchew/20190423160608.7519-1-thuth@redhat.com
 * [new tag]               patchew/20190423212246.3542-1-ehabkost@redhat.com -> patchew/20190423212246.3542-1-ehabkost@redhat.com
Switched to a new branch 'test'
29e2716a80 qmp: Add deprecation information to query-machines
34cb9a3b70 machine: Use SupportStatusInfo for deprecation info
b44b7a4718 qapi: SupportStatusInfo struct

=== OUTPUT BEGIN ===
1/3 Checking commit b44b7a47188e (qapi: SupportStatusInfo struct)
2/3 Checking commit 34cb9a3b7083 (machine: Use SupportStatusInfo for deprecation info)
3/3 Checking commit 29e2716a80c8 (qmp: Add deprecation information to query-machines)
WARNING: added, moved or deleted file(s), does MAINTAINERS need updating?
#37: 
new file mode 100644

ERROR: line over 90 characters
#65: FILE: tests/acceptance/query_machines.py:24:
+                                "Deprecated machine (%s) must have status-message or suggested-alternative" % (machine['name']))

ERROR: line over 90 characters
#68: FILE: tests/acceptance/query_machines.py:27:
+                                "suggested-alternative of %s must point to a valid machine type" % (machine['name']))

total: 2 errors, 1 warnings, 55 lines checked

Patch 3/3 has style problems, please review.  If any of these errors
are false positives report them to the maintainer, see
CHECKPATCH in MAINTAINERS.

=== OUTPUT END ===

Test command exited with code: 1


The full log is available at
http://patchew.org/logs/20190423212246.3542-1-ehabkost@redhat.com/testing.checkpatch/?type=message.
---
Email generated automatically by Patchew [https://patchew.org/].
Please send your feedback to patchew-devel@redhat.com
Thomas Huth April 24, 2019, 7:56 a.m. UTC | #2
On 23/04/2019 23.22, Eduardo Habkost wrote:
> This series adds machine type deprecation information to the
> output of the `query-machines` QMP command.  With this, libvirt
> and management software will be able to show this information to
> users and/or suggest changes to VM configuration to avoid
> deprecated machine types.
> 
> Eduardo Habkost (3):
>   qapi: SupportStatusInfo struct
>   machine: Use SupportStatusInfo for deprecation info
>   qmp: Add deprecation information to query-machines
> 
>  qapi/common.json                   | 24 ++++++++++++++++++++++++
>  qapi/misc.json                     |  5 ++++-
>  include/hw/boards.h                |  7 ++++---
>  hw/i386/pc_piix.c                  |  4 +++-
>  hw/ppc/prep.c                      |  4 +++-
>  vl.c                               | 19 +++++++++++++++----
>  tests/acceptance/query_machines.py | 27 +++++++++++++++++++++++++++
>  7 files changed, 80 insertions(+), 10 deletions(-)
>  create mode 100644 tests/acceptance/query_machines.py

Good idea, but some questions come to my mind:

- What about devices? IIRC Gerd wrote a patch series last year that does
  something similar for devices... It would be good to synchronize the
  work, so that we do not have two completely interfaces between devices
  and machines here in the end...

- Is deprecation as a status enough, or do we want to carry more
  information here? E.g. is the machine maintained or orphan? Is it
  stable or rather experimental? And didn't Gerd have also some
  patches for this last year? ... yes, I think it was this series here:
  http://lists.gnu.org/archive/html/qemu-ppc/2018-11/msg00039.html
  ... actually, I like that idea with QemuSupportState... maybe you
  could base your work on that series instead?

 Thomas
Michal Privoznik April 24, 2019, 8:23 a.m. UTC | #3
On 4/23/19 11:22 PM, Eduardo Habkost wrote:
> This series adds machine type deprecation information to the
> output of the `query-machines` QMP command.  With this, libvirt
> and management software will be able to show this information to
> users and/or suggest changes to VM configuration to avoid
> deprecated machine types.
> 
> Eduardo Habkost (3):
>    qapi: SupportStatusInfo struct
>    machine: Use SupportStatusInfo for deprecation info
>    qmp: Add deprecation information to query-machines
> 
>   qapi/common.json                   | 24 ++++++++++++++++++++++++
>   qapi/misc.json                     |  5 ++++-
>   include/hw/boards.h                |  7 ++++---
>   hw/i386/pc_piix.c                  |  4 +++-
>   hw/ppc/prep.c                      |  4 +++-
>   vl.c                               | 19 +++++++++++++++----
>   tests/acceptance/query_machines.py | 27 +++++++++++++++++++++++++++
>   7 files changed, 80 insertions(+), 10 deletions(-)
>   create mode 100644 tests/acceptance/query_machines.py
> 

ACK.

Now we need to think how to deal with this in libvirt. Whether it's 
sufficient to expose this in capabilities XML or libvirt needs to do 
something more.

Michal
Daniel P. Berrangé April 24, 2019, 8:31 a.m. UTC | #4
On Wed, Apr 24, 2019 at 09:56:53AM +0200, Thomas Huth wrote:
> On 23/04/2019 23.22, Eduardo Habkost wrote:
> > This series adds machine type deprecation information to the
> > output of the `query-machines` QMP command.  With this, libvirt
> > and management software will be able to show this information to
> > users and/or suggest changes to VM configuration to avoid
> > deprecated machine types.
> > 
> > Eduardo Habkost (3):
> >   qapi: SupportStatusInfo struct
> >   machine: Use SupportStatusInfo for deprecation info
> >   qmp: Add deprecation information to query-machines
> > 
> >  qapi/common.json                   | 24 ++++++++++++++++++++++++
> >  qapi/misc.json                     |  5 ++++-
> >  include/hw/boards.h                |  7 ++++---
> >  hw/i386/pc_piix.c                  |  4 +++-
> >  hw/ppc/prep.c                      |  4 +++-
> >  vl.c                               | 19 +++++++++++++++----
> >  tests/acceptance/query_machines.py | 27 +++++++++++++++++++++++++++
> >  7 files changed, 80 insertions(+), 10 deletions(-)
> >  create mode 100644 tests/acceptance/query_machines.py
> 
> Good idea, but some questions come to my mind:
> 
> - What about devices? IIRC Gerd wrote a patch series last year that does
>   something similar for devices... It would be good to synchronize the
>   work, so that we do not have two completely interfaces between devices
>   and machines here in the end...

Might as well add  CPU models to that too as there's plenty of awful
CPUs no one should use with KVM. The tricky thing is they are fine
/ sensible with TCG still.

> 
> - Is deprecation as a status enough, or do we want to carry more
>   information here? E.g. is the machine maintained or orphan? Is it
>   stable or rather experimental? And didn't Gerd have also some
>   patches for this last year? ... yes, I think it was this series here:
>   http://lists.gnu.org/archive/html/qemu-ppc/2018-11/msg00039.html
>   ... actually, I like that idea with QemuSupportState... maybe you
>   could base your work on that series instead?


Regards,
Daniel
Eduardo Habkost April 24, 2019, 6:10 p.m. UTC | #5
On Wed, Apr 24, 2019 at 09:56:53AM +0200, Thomas Huth wrote:
> On 23/04/2019 23.22, Eduardo Habkost wrote:
> > This series adds machine type deprecation information to the
> > output of the `query-machines` QMP command.  With this, libvirt
> > and management software will be able to show this information to
> > users and/or suggest changes to VM configuration to avoid
> > deprecated machine types.
> > 
> > Eduardo Habkost (3):
> >   qapi: SupportStatusInfo struct
> >   machine: Use SupportStatusInfo for deprecation info
> >   qmp: Add deprecation information to query-machines
> > 
> >  qapi/common.json                   | 24 ++++++++++++++++++++++++
> >  qapi/misc.json                     |  5 ++++-
> >  include/hw/boards.h                |  7 ++++---
> >  hw/i386/pc_piix.c                  |  4 +++-
> >  hw/ppc/prep.c                      |  4 +++-
> >  vl.c                               | 19 +++++++++++++++----
> >  tests/acceptance/query_machines.py | 27 +++++++++++++++++++++++++++
> >  7 files changed, 80 insertions(+), 10 deletions(-)
> >  create mode 100644 tests/acceptance/query_machines.py
> 
> Good idea, but some questions come to my mind:
> 
> - What about devices? IIRC Gerd wrote a patch series last year that does
>   something similar for devices... It would be good to synchronize the
>   work, so that we do not have two completely interfaces between devices
>   and machines here in the end...

My plan is to support this on devices, too.  I even had a version
where documentation of SupportStatusInfo mentioned device types,
but I decided to leave that out until we actually implement a
device deprecation info API.

> 
> - Is deprecation as a status enough, or do we want to carry more
>   information here? E.g. is the machine maintained or orphan? Is it
>   stable or rather experimental? And didn't Gerd have also some
>   patches for this last year? ... yes, I think it was this series here:
>   http://lists.gnu.org/archive/html/qemu-ppc/2018-11/msg00039.html
>   ... actually, I like that idea with QemuSupportState... maybe you
>   could base your work on that series instead?

We might want to carry more information eventually.  The
possibility of extending the data later is the main reason I
called the struct SupportStatusInfo and not just DeprecationInfo.

But I'd really like us to export the existing information first,
before extending the data.  Having important data available
through stderr and not QMP is a bug.  Tracking additional data
may be desirable, but it would be an additional feature.
Eduardo Habkost April 24, 2019, 6:14 p.m. UTC | #6
On Wed, Apr 24, 2019 at 09:31:13AM +0100, Daniel P. Berrangé wrote:
> On Wed, Apr 24, 2019 at 09:56:53AM +0200, Thomas Huth wrote:
> > On 23/04/2019 23.22, Eduardo Habkost wrote:
> > > This series adds machine type deprecation information to the
> > > output of the `query-machines` QMP command.  With this, libvirt
> > > and management software will be able to show this information to
> > > users and/or suggest changes to VM configuration to avoid
> > > deprecated machine types.
> > > 
> > > Eduardo Habkost (3):
> > >   qapi: SupportStatusInfo struct
> > >   machine: Use SupportStatusInfo for deprecation info
> > >   qmp: Add deprecation information to query-machines
> > > 
> > >  qapi/common.json                   | 24 ++++++++++++++++++++++++
> > >  qapi/misc.json                     |  5 ++++-
> > >  include/hw/boards.h                |  7 ++++---
> > >  hw/i386/pc_piix.c                  |  4 +++-
> > >  hw/ppc/prep.c                      |  4 +++-
> > >  vl.c                               | 19 +++++++++++++++----
> > >  tests/acceptance/query_machines.py | 27 +++++++++++++++++++++++++++
> > >  7 files changed, 80 insertions(+), 10 deletions(-)
> > >  create mode 100644 tests/acceptance/query_machines.py
> > 
> > Good idea, but some questions come to my mind:
> > 
> > - What about devices? IIRC Gerd wrote a patch series last year that does
> >   something similar for devices... It would be good to synchronize the
> >   work, so that we do not have two completely interfaces between devices
> >   and machines here in the end...
> 
> Might as well add  CPU models to that too as there's plenty of awful
> CPUs no one should use with KVM. The tricky thing is they are fine
> / sensible with TCG still.

Right.  We also have devices that are supported in some machines
but not in others.  We will probably need a separate API that
takes a (device type, machine type, accelerator) tuple as input.
Thomas Huth April 25, 2019, 7:38 a.m. UTC | #7
On 24/04/2019 20.10, Eduardo Habkost wrote:
> On Wed, Apr 24, 2019 at 09:56:53AM +0200, Thomas Huth wrote:
>> On 23/04/2019 23.22, Eduardo Habkost wrote:
>>> This series adds machine type deprecation information to the
>>> output of the `query-machines` QMP command.  With this, libvirt
>>> and management software will be able to show this information to
>>> users and/or suggest changes to VM configuration to avoid
>>> deprecated machine types.
>>>
>>> Eduardo Habkost (3):
>>>   qapi: SupportStatusInfo struct
>>>   machine: Use SupportStatusInfo for deprecation info
>>>   qmp: Add deprecation information to query-machines
>>>
>>>  qapi/common.json                   | 24 ++++++++++++++++++++++++
>>>  qapi/misc.json                     |  5 ++++-
>>>  include/hw/boards.h                |  7 ++++---
>>>  hw/i386/pc_piix.c                  |  4 +++-
>>>  hw/ppc/prep.c                      |  4 +++-
>>>  vl.c                               | 19 +++++++++++++++----
>>>  tests/acceptance/query_machines.py | 27 +++++++++++++++++++++++++++
>>>  7 files changed, 80 insertions(+), 10 deletions(-)
>>>  create mode 100644 tests/acceptance/query_machines.py
>>
>> Good idea, but some questions come to my mind:
>>
>> - What about devices? IIRC Gerd wrote a patch series last year that does
>>   something similar for devices... It would be good to synchronize the
>>   work, so that we do not have two completely interfaces between devices
>>   and machines here in the end...
> 
> My plan is to support this on devices, too.  I even had a version
> where documentation of SupportStatusInfo mentioned device types,
> but I decided to leave that out until we actually implement a
> device deprecation info API.
> 
>>
>> - Is deprecation as a status enough, or do we want to carry more
>>   information here? E.g. is the machine maintained or orphan? Is it
>>   stable or rather experimental? And didn't Gerd have also some
>>   patches for this last year? ... yes, I think it was this series here:
>>   http://lists.gnu.org/archive/html/qemu-ppc/2018-11/msg00039.html
>>   ... actually, I like that idea with QemuSupportState... maybe you
>>   could base your work on that series instead?
> 
> We might want to carry more information eventually.  The
> possibility of extending the data later is the main reason I
> called the struct SupportStatusInfo and not just DeprecationInfo.

Ok. I was just a little bit afraid that we define an interface here that
we have to change again completely once we want to carry more
information. For example whether "deprecated" should be a "bool" here,
or rather one of the "enum" entries like in Gerd's series. But after
reading through https://patchwork.kernel.org/patch/10660677/ again, I
think I agree that "deprecated" is orthogonal to the support state, e.g.
a device could still be supported (in the sense that there is a
maintainer for it), while it has been marked as "deprecated" already.
So no more objections from my side here.

 Thomas
Daniel P. Berrangé April 30, 2019, 10:11 a.m. UTC | #8
On Wed, Apr 24, 2019 at 03:10:49PM -0300, Eduardo Habkost wrote:
> On Wed, Apr 24, 2019 at 09:56:53AM +0200, Thomas Huth wrote:
> > On 23/04/2019 23.22, Eduardo Habkost wrote:
> > > This series adds machine type deprecation information to the
> > > output of the `query-machines` QMP command.  With this, libvirt
> > > and management software will be able to show this information to
> > > users and/or suggest changes to VM configuration to avoid
> > > deprecated machine types.
> > > 
> > > Eduardo Habkost (3):
> > >   qapi: SupportStatusInfo struct
> > >   machine: Use SupportStatusInfo for deprecation info
> > >   qmp: Add deprecation information to query-machines
> > > 
> > >  qapi/common.json                   | 24 ++++++++++++++++++++++++
> > >  qapi/misc.json                     |  5 ++++-
> > >  include/hw/boards.h                |  7 ++++---
> > >  hw/i386/pc_piix.c                  |  4 +++-
> > >  hw/ppc/prep.c                      |  4 +++-
> > >  vl.c                               | 19 +++++++++++++++----
> > >  tests/acceptance/query_machines.py | 27 +++++++++++++++++++++++++++
> > >  7 files changed, 80 insertions(+), 10 deletions(-)
> > >  create mode 100644 tests/acceptance/query_machines.py
> > 
> > Good idea, but some questions come to my mind:
> > 
> > - What about devices? IIRC Gerd wrote a patch series last year that does
> >   something similar for devices... It would be good to synchronize the
> >   work, so that we do not have two completely interfaces between devices
> >   and machines here in the end...
> 
> My plan is to support this on devices, too.  I even had a version
> where documentation of SupportStatusInfo mentioned device types,
> but I decided to leave that out until we actually implement a
> device deprecation info API.

We need it on CPU models, and on CPU features too potentially. Essentially
we should consider it applicable to anything we report in QMP.


Regards,
Daniel
Markus Armbruster May 7, 2019, 5:07 a.m. UTC | #9
Eduardo Habkost <ehabkost@redhat.com> writes:

> This series adds machine type deprecation information to the
> output of the `query-machines` QMP command.  With this, libvirt
> and management software will be able to show this information to
> users and/or suggest changes to VM configuration to avoid
> deprecated machine types.

This overlaps with something I want to try, namely using Kevin's
proposed QAPI feature flags for deprecation markings.  Let's compare the
two.

To mark something as deprecated with your patches, you add a
@support-status member somewhere, where "somewhere" is related to
"something" by "provides information on".

Example: MachineInfo (returned by query-machines) provides information
on possible values of -machine parameter type.  If -machine was
QAPIfied, it would provide information on possible values of a QAPI
object type's member.  The type might be anonymous.  The member should
be an enum (we currently use 'str' in MachineInfo).

Example: say we want to deprecate block driver "vfat",
i.e. BlockdevDriver member @vfat.  Type BlockdevDriver is used in
multiple places; let's ignore all but BlockdevOptions.  We need to add
@support-status to something that provides information on
BlockdevDriver, or maybe on BlockdevOptions.  There is no ad hoc query
providing information on either of the two, because QAPI/QMP
introspection has been sufficient.  What now?

Can we add deprecation information to (general) QAPI/QMP introspection
instead of ad hoc queries?

Kevin's proposed QAPI feature flags[*] extend the QAPI language so that
struct types can optionally have a list of feature flags, which are
strings.  Struct types suffice for his immediate needs.  I'd like to use
feature flags to mark deprecation by tacking a "deprecated" feature onto
whatever is deprecated.  This obviously needs feature support for
everything we want to be able to deprecate: commands, and events, as
well as members of enum and object types.

Example: to deprecate block driver "vfat", add feature "deprecated" to
BlockdevDriver member @vfat.

Unlike your patches, this does not require finding a "somewhere" that
provides information on "something".  You simply tack "deprecated" right
onto "something".

Your patches provide more information, however: human-readable messages.

Food for thought :)


[*] Hiding in
Subject: [PATCH 0/4] file-posix: Add dynamic-auto-read-only QAPI feature
Date: Mon,  8 Apr 2019 16:35:39 +0200
Message-Id: <20190408143543.3982-1-kwolf@redhat.com>
Eduardo Habkost May 7, 2019, 4:18 p.m. UTC | #10
On Tue, May 07, 2019 at 07:07:04AM +0200, Markus Armbruster wrote:
> Eduardo Habkost <ehabkost@redhat.com> writes:
> 
> > This series adds machine type deprecation information to the
> > output of the `query-machines` QMP command.  With this, libvirt
> > and management software will be able to show this information to
> > users and/or suggest changes to VM configuration to avoid
> > deprecated machine types.
> 
> This overlaps with something I want to try, namely using Kevin's
> proposed QAPI feature flags for deprecation markings.  Let's compare the
> two.
> 
> To mark something as deprecated with your patches, you add a
> @support-status member somewhere, where "somewhere" is related to
> "something" by "provides information on".
> 
> Example: MachineInfo (returned by query-machines) provides information
> on possible values of -machine parameter type.  If -machine was
> QAPIfied, it would provide information on possible values of a QAPI
> object type's member.  The type might be anonymous.  The member should
> be an enum (we currently use 'str' in MachineInfo).

QAPIfying -machine, -cpu, and -device would be wonderful.

> 
> Example: say we want to deprecate block driver "vfat",
> i.e. BlockdevDriver member @vfat.  Type BlockdevDriver is used in
> multiple places; let's ignore all but BlockdevOptions.  We need to add
> @support-status to something that provides information on
> BlockdevDriver, or maybe on BlockdevOptions.  There is no ad hoc query
> providing information on either of the two, because QAPI/QMP
> introspection has been sufficient.  What now?
> 
> Can we add deprecation information to (general) QAPI/QMP introspection

Yes, we can.  I think it's a good idea.  But:

> instead of ad hoc queries?

I'm not sure about the "instead of" part.  I don't want perfect
to be the enemy of done, and I don't want QAPIfication of
-machine to be a requirement to start reporting machine type
deprecation information.

> 
> Kevin's proposed QAPI feature flags[*] extend the QAPI language so that
> struct types can optionally have a list of feature flags, which are
> strings.  Struct types suffice for his immediate needs.  I'd like to use
> feature flags to mark deprecation by tacking a "deprecated" feature onto
> whatever is deprecated.  This obviously needs feature support for
> everything we want to be able to deprecate: commands, and events, as
> well as members of enum and object types.
> 
> Example: to deprecate block driver "vfat", add feature "deprecated" to
> BlockdevDriver member @vfat.
> 
> Unlike your patches, this does not require finding a "somewhere" that
> provides information on "something".  You simply tack "deprecated" right
> onto "something".
> 
> Your patches provide more information, however: human-readable messages.

It also includes a machine-friendly suggested alternative (which
I think is even more important that the human-readable message).

We could extend QAPI introspection to return that if necessary,
right?


> 
> Food for thought :)
> 
> 
> [*] Hiding in
> Subject: [PATCH 0/4] file-posix: Add dynamic-auto-read-only QAPI feature
> Date: Mon,  8 Apr 2019 16:35:39 +0200
> Message-Id: <20190408143543.3982-1-kwolf@redhat.com>
Markus Armbruster May 8, 2019, 9:16 a.m. UTC | #11
Eduardo Habkost <ehabkost@redhat.com> writes:

> On Tue, May 07, 2019 at 07:07:04AM +0200, Markus Armbruster wrote:
>> Eduardo Habkost <ehabkost@redhat.com> writes:
>> 
>> > This series adds machine type deprecation information to the
>> > output of the `query-machines` QMP command.  With this, libvirt
>> > and management software will be able to show this information to
>> > users and/or suggest changes to VM configuration to avoid
>> > deprecated machine types.
>> 
>> This overlaps with something I want to try, namely using Kevin's
>> proposed QAPI feature flags for deprecation markings.  Let's compare the
>> two.
>> 
>> To mark something as deprecated with your patches, you add a
>> @support-status member somewhere, where "somewhere" is related to
>> "something" by "provides information on".
>> 
>> Example: MachineInfo (returned by query-machines) provides information
>> on possible values of -machine parameter type.  If -machine was
>> QAPIfied, it would provide information on possible values of a QAPI
>> object type's member.  The type might be anonymous.  The member should
>> be an enum (we currently use 'str' in MachineInfo).
>
> QAPIfying -machine, -cpu, and -device would be wonderful.
>
>> 
>> Example: say we want to deprecate block driver "vfat",
>> i.e. BlockdevDriver member @vfat.  Type BlockdevDriver is used in
>> multiple places; let's ignore all but BlockdevOptions.  We need to add
>> @support-status to something that provides information on
>> BlockdevDriver, or maybe on BlockdevOptions.  There is no ad hoc query
>> providing information on either of the two, because QAPI/QMP
>> introspection has been sufficient.  What now?
>> 
>> Can we add deprecation information to (general) QAPI/QMP introspection
>
> Yes, we can.  I think it's a good idea.  But:
>
>> instead of ad hoc queries?
>
> I'm not sure about the "instead of" part.  I don't want perfect
> to be the enemy of done, and I don't want QAPIfication of
> -machine to be a requirement to start reporting machine type
> deprecation information.

Valid point.  Still, I believe we should at least try to predict how the
pieces we create now would fit with the pieces we plan to create later
on.

Note that full QAPIfication of -machine isn't necessary to make QAPI
feature "deprecated" work for machine types.  Turning MachineInfo member
@name into an enum, so we can tack "deprecated" onto its values, would
suffice.

Such a QAPIfication of machine types is still hard: QOM types are
defined at compile time just like the QAPI schema, but their definition
is distributed, and collected into one place only at run time.  I
discussed this on slide 39 of my "QEMU interface introspection: From
hacks to solutions" talk (KVM Form 2015).  Just for device_add, but it's
just a special case of QOM.  Choices listed there:

* Collect drivers at compile time? Hard...
* Make QAPI schema dynamic? Hard...
* Forgo driver-specific arguments in schema?
  Defeats introspection...

I'd like to add to the last item:

  Provide QOM introspection on par with QAPI schema introspection

The QOM introspection we have (qom-list-types etc. is not on par.

Back to exposing machine type deprecation.

I'm doubtful your proposed solution can be applied widely.  It relies on
adding @support-status to something that provides information on
whatever is deprecated.  The initial use is with a something that is an
ad hoc query, namely query-machines.  To use it, the management
application needs to understand what query-machines' @support-status
applies to.  Certainly feasible.  But I fear every use will be a special
case.  Furthermore, a suitable ad hoc query need not exist.  What then?
Create suitable ad hoc queries just for communicating deprecation?

Instead, I'd like us to think about a more genral solution.  Or perhaps
two: one for properly QAPIfied stuff, and one for QOM.

>> Kevin's proposed QAPI feature flags[*] extend the QAPI language so that
>> struct types can optionally have a list of feature flags, which are
>> strings.  Struct types suffice for his immediate needs.  I'd like to use
>> feature flags to mark deprecation by tacking a "deprecated" feature onto
>> whatever is deprecated.  This obviously needs feature support for
>> everything we want to be able to deprecate: commands, and events, as
>> well as members of enum and object types.
>> 
>> Example: to deprecate block driver "vfat", add feature "deprecated" to
>> BlockdevDriver member @vfat.
>> 
>> Unlike your patches, this does not require finding a "somewhere" that
>> provides information on "something".  You simply tack "deprecated" right
>> onto "something".
>> 
>> Your patches provide more information, however: human-readable messages.
>
> It also includes a machine-friendly suggested alternative (which
> I think is even more important that the human-readable message).

I agree we should point to a preferred replacement whenever we deprecate
something.

We have to do it in documentation.  And we generally do, in
qemu-deprecated.texi.

How useful would doing it in QMP as well be?  Depends on what management
applications can do with the additional information.

> We could extend QAPI introspection to return that if necessary,
> right?

I'm confident we can come up with *something*.  It might kill the neat
and simple "use QAPI features to communicate deprecation" idea, though.

>> Food for thought :)
>> 
>> 
>> [*] Hiding in
>> Subject: [PATCH 0/4] file-posix: Add dynamic-auto-read-only QAPI feature
>> Date: Mon,  8 Apr 2019 16:35:39 +0200
>> Message-Id: <20190408143543.3982-1-kwolf@redhat.com>
Eduardo Habkost May 8, 2019, 8:28 p.m. UTC | #12
On Wed, May 08, 2019 at 11:16:50AM +0200, Markus Armbruster wrote:
> Eduardo Habkost <ehabkost@redhat.com> writes:
> 
> > On Tue, May 07, 2019 at 07:07:04AM +0200, Markus Armbruster wrote:
> >> Eduardo Habkost <ehabkost@redhat.com> writes:
> >> 
> >> > This series adds machine type deprecation information to the
> >> > output of the `query-machines` QMP command.  With this, libvirt
> >> > and management software will be able to show this information to
> >> > users and/or suggest changes to VM configuration to avoid
> >> > deprecated machine types.
> >> 
> >> This overlaps with something I want to try, namely using Kevin's
> >> proposed QAPI feature flags for deprecation markings.  Let's compare the
> >> two.
> >> 
> >> To mark something as deprecated with your patches, you add a
> >> @support-status member somewhere, where "somewhere" is related to
> >> "something" by "provides information on".
> >> 
> >> Example: MachineInfo (returned by query-machines) provides information
> >> on possible values of -machine parameter type.  If -machine was
> >> QAPIfied, it would provide information on possible values of a QAPI
> >> object type's member.  The type might be anonymous.  The member should
> >> be an enum (we currently use 'str' in MachineInfo).
> >
> > QAPIfying -machine, -cpu, and -device would be wonderful.
> >
> >> 
> >> Example: say we want to deprecate block driver "vfat",
> >> i.e. BlockdevDriver member @vfat.  Type BlockdevDriver is used in
> >> multiple places; let's ignore all but BlockdevOptions.  We need to add
> >> @support-status to something that provides information on
> >> BlockdevDriver, or maybe on BlockdevOptions.  There is no ad hoc query
> >> providing information on either of the two, because QAPI/QMP
> >> introspection has been sufficient.  What now?
> >> 
> >> Can we add deprecation information to (general) QAPI/QMP introspection
> >
> > Yes, we can.  I think it's a good idea.  But:
> >
> >> instead of ad hoc queries?
> >
> > I'm not sure about the "instead of" part.  I don't want perfect
> > to be the enemy of done, and I don't want QAPIfication of
> > -machine to be a requirement to start reporting machine type
> > deprecation information.
> 
> Valid point.  Still, I believe we should at least try to predict how the
> pieces we create now would fit with the pieces we plan to create later
> on.

Sure.

> 
> Note that full QAPIfication of -machine isn't necessary to make QAPI
> feature "deprecated" work for machine types.  Turning MachineInfo member
> @name into an enum, so we can tack "deprecated" onto its values, would
> suffice.
> 
> Such a QAPIfication of machine types is still hard: QOM types are
> defined at compile time just like the QAPI schema, but their definition
> is distributed, and collected into one place only at run time.  I
> discussed this on slide 39 of my "QEMU interface introspection: From
> hacks to solutions" talk (KVM Form 2015).  Just for device_add, but it's
> just a special case of QOM.  Choices listed there:
> 
> * Collect drivers at compile time? Hard...
> * Make QAPI schema dynamic? Hard...
> * Forgo driver-specific arguments in schema?
>   Defeats introspection...
> 
> I'd like to add to the last item:
> 
>   Provide QOM introspection on par with QAPI schema introspection
> 
> The QOM introspection we have (qom-list-types etc. is not on par.

Agreed, but do we really want to do it?  We have been avoiding
exposing QOM internals to the outside on purpose.  I believe
there are at least two reasons for that:

1) Not every QOM type/property is supposed to be visible to the
   outside (and nobody really knows what's the full set of
   supported external QOM interfaces);
2) QAPI is our preferred interface interface specification/introspection
   mechanism.

> 
> Back to exposing machine type deprecation.
> 
> I'm doubtful your proposed solution can be applied widely.  It relies on
> adding @support-status to something that provides information on
> whatever is deprecated.  The initial use is with a something that is an
> ad hoc query, namely query-machines.  To use it, the management
> application needs to understand what query-machines' @support-status
> applies to.  Certainly feasible.  But I fear every use will be a special
> case.  Furthermore, a suitable ad hoc query need not exist.  What then?
> Create suitable ad hoc queries just for communicating deprecation?
> 
> Instead, I'd like us to think about a more genral solution.  Or perhaps
> two: one for properly QAPIfied stuff, and one for QOM.

Should we really spend our time designing a second solution, if
we could build this on top of QAPI abstractions?  Making the QAPI
schema dynamic might be hard, but reinventing QAPI and
maintaining the two systems in parallel seems harder.


> 
> >> Kevin's proposed QAPI feature flags[*] extend the QAPI language so that
> >> struct types can optionally have a list of feature flags, which are
> >> strings.  Struct types suffice for his immediate needs.  I'd like to use
> >> feature flags to mark deprecation by tacking a "deprecated" feature onto
> >> whatever is deprecated.  This obviously needs feature support for
> >> everything we want to be able to deprecate: commands, and events, as
> >> well as members of enum and object types.
> >> 
> >> Example: to deprecate block driver "vfat", add feature "deprecated" to
> >> BlockdevDriver member @vfat.
> >> 
> >> Unlike your patches, this does not require finding a "somewhere" that
> >> provides information on "something".  You simply tack "deprecated" right
> >> onto "something".
> >> 
> >> Your patches provide more information, however: human-readable messages.
> >
> > It also includes a machine-friendly suggested alternative (which
> > I think is even more important that the human-readable message).
> 
> I agree we should point to a preferred replacement whenever we deprecate
> something.
> 
> We have to do it in documentation.  And we generally do, in
> qemu-deprecated.texi.
> 
> How useful would doing it in QMP as well be?  Depends on what management
> applications can do with the additional information.

I expect it to be useful for things that have obvious
replacements, like old machine type or CPU model versions.

> 
> > We could extend QAPI introspection to return that if necessary,
> > right?
> 
> I'm confident we can come up with *something*.  It might kill the neat
> and simple "use QAPI features to communicate deprecation" idea, though.
> 

If something is important enough to be communicated through
stderr, it's important enough to be communicated through QMP.
Is that enough reason to provide something more complex?

Do we need QAPI features to be just strings?  Can't they be a
more complex type, like a QAPI alternate?


> >> Food for thought :)
> >> 
> >> 
> >> [*] Hiding in
> >> Subject: [PATCH 0/4] file-posix: Add dynamic-auto-read-only QAPI feature
> >> Date: Mon,  8 Apr 2019 16:35:39 +0200
> >> Message-Id: <20190408143543.3982-1-kwolf@redhat.com>
Markus Armbruster May 9, 2019, 8:31 a.m. UTC | #13
We've wandered into the QAPI vs. QOM swamp.  Cc: Paolo.

Eduardo Habkost <ehabkost@redhat.com> writes:

> On Wed, May 08, 2019 at 11:16:50AM +0200, Markus Armbruster wrote:
>> Eduardo Habkost <ehabkost@redhat.com> writes:
>> 
>> > On Tue, May 07, 2019 at 07:07:04AM +0200, Markus Armbruster wrote:
>> >> Eduardo Habkost <ehabkost@redhat.com> writes:
>> >> 
>> >> > This series adds machine type deprecation information to the
>> >> > output of the `query-machines` QMP command.  With this, libvirt
>> >> > and management software will be able to show this information to
>> >> > users and/or suggest changes to VM configuration to avoid
>> >> > deprecated machine types.
>> >> 
>> >> This overlaps with something I want to try, namely using Kevin's
>> >> proposed QAPI feature flags for deprecation markings.  Let's compare the
>> >> two.
>> >> 
>> >> To mark something as deprecated with your patches, you add a
>> >> @support-status member somewhere, where "somewhere" is related to
>> >> "something" by "provides information on".
>> >> 
>> >> Example: MachineInfo (returned by query-machines) provides information
>> >> on possible values of -machine parameter type.  If -machine was
>> >> QAPIfied, it would provide information on possible values of a QAPI
>> >> object type's member.  The type might be anonymous.  The member should
>> >> be an enum (we currently use 'str' in MachineInfo).
>> >
>> > QAPIfying -machine, -cpu, and -device would be wonderful.
>> >
>> >> 
>> >> Example: say we want to deprecate block driver "vfat",
>> >> i.e. BlockdevDriver member @vfat.  Type BlockdevDriver is used in
>> >> multiple places; let's ignore all but BlockdevOptions.  We need to add
>> >> @support-status to something that provides information on
>> >> BlockdevDriver, or maybe on BlockdevOptions.  There is no ad hoc query
>> >> providing information on either of the two, because QAPI/QMP
>> >> introspection has been sufficient.  What now?
>> >> 
>> >> Can we add deprecation information to (general) QAPI/QMP introspection
>> >
>> > Yes, we can.  I think it's a good idea.  But:
>> >
>> >> instead of ad hoc queries?
>> >
>> > I'm not sure about the "instead of" part.  I don't want perfect
>> > to be the enemy of done, and I don't want QAPIfication of
>> > -machine to be a requirement to start reporting machine type
>> > deprecation information.
>> 
>> Valid point.  Still, I believe we should at least try to predict how the
>> pieces we create now would fit with the pieces we plan to create later
>> on.
>
> Sure.
>
>> 
>> Note that full QAPIfication of -machine isn't necessary to make QAPI
>> feature "deprecated" work for machine types.  Turning MachineInfo member
>> @name into an enum, so we can tack "deprecated" onto its values, would
>> suffice.
>> 
>> Such a QAPIfication of machine types is still hard: QOM types are
>> defined at compile time just like the QAPI schema, but their definition
>> is distributed, and collected into one place only at run time.  I
>> discussed this on slide 39 of my "QEMU interface introspection: From
>> hacks to solutions" talk (KVM Form 2015).  Just for device_add, but it's
>> just a special case of QOM.  Choices listed there:
>> 
>> * Collect drivers at compile time? Hard...
>> * Make QAPI schema dynamic? Hard...
>> * Forgo driver-specific arguments in schema?
>>   Defeats introspection...
>> 
>> I'd like to add to the last item:
>> 
>>   Provide QOM introspection on par with QAPI schema introspection
>> 
>> The QOM introspection we have (qom-list-types etc. is not on par.
>
> Agreed, but do we really want to do it?  We have been avoiding
> exposing QOM internals to the outside on purpose.  I believe
> there are at least two reasons for that:
>
> 1) Not every QOM type/property is supposed to be visible to the
>    outside

True.

However, the parts of QOM exposed via device_add and object-add are
definitely part of the stable external interface (unless explicitly
marked unstable).

>            (and nobody really knows what's the full set of
>    supported external QOM interfaces);

Also true.  And terrible.

> 2) QAPI is our preferred interface interface specification/introspection
>    mechanism.

When preferences and requirements collide, preferences tend to get run
over.

The QAPI schema is *declarative*: the schema declares QAPI objects and
properties.  We generate C from the schema, which we then compile and
link into QEMU.

QOM is by design *imperative*: we execute compiled C at QEMU run-time to
define QOM objects and properties.  Maximizes flexibility.  See also
Turing tarpit.

No matter how much we'd prefer to use QAPI to specify external
interfaces to QOM, we can't without making QAPI much more dynamic or QOM
much more static.  Either is hard.  Quite possibly infeasible.

We could try to extend QAPI/QMP introspection to somehow merge in
additional information at run-time[1].  Could be regarded as a limited
way to make QAPI more dynamic.  This is in the "vague idea, not sure
it's feasible" stage.

We could try to rearchitect QOM so that you can optionally specify QOM
stuff in the QAPI schema, then require that for QOM stuff that's part of
a stable external interface.  Same "vague idea" stage, feasibility even
more doubtful.

If I remember correctly, Paolo suggested to abandon the idea to have
query-qmp-schema cover device_add as infeasible.  Instead, cover it
separately.

I think this separate introspection mechanism should be as similar to
query-qmp-schema as practical.  It should cover not just device_add, but
also object-add, and whatever other external interface to QOM we deem
stable.

Should there be more stable external interfaces that can't be defined in
the QAPI schema, we might want to cover them in this second
introspection mechanism, too.

>> Back to exposing machine type deprecation.
>> 
>> I'm doubtful your proposed solution can be applied widely.  It relies on
>> adding @support-status to something that provides information on
>> whatever is deprecated.  The initial use is with a something that is an
>> ad hoc query, namely query-machines.  To use it, the management
>> application needs to understand what query-machines' @support-status
>> applies to.  Certainly feasible.  But I fear every use will be a special
>> case.  Furthermore, a suitable ad hoc query need not exist.  What then?
>> Create suitable ad hoc queries just for communicating deprecation?
>> 
>> Instead, I'd like us to think about a more genral solution.  Or perhaps
>> two: one for properly QAPIfied stuff, and one for QOM.
>
> Should we really spend our time designing a second solution, if
> we could build this on top of QAPI abstractions?  Making the QAPI
> schema dynamic might be hard, but reinventing QAPI and
> maintaining the two systems in parallel seems harder.

I'd love to see a practical way forward that keeps QAPI/QMP
introspection the only game in town.

>> >> Kevin's proposed QAPI feature flags[*] extend the QAPI language so that
>> >> struct types can optionally have a list of feature flags, which are
>> >> strings.  Struct types suffice for his immediate needs.  I'd like to use
>> >> feature flags to mark deprecation by tacking a "deprecated" feature onto
>> >> whatever is deprecated.  This obviously needs feature support for
>> >> everything we want to be able to deprecate: commands, and events, as
>> >> well as members of enum and object types.
>> >> 
>> >> Example: to deprecate block driver "vfat", add feature "deprecated" to
>> >> BlockdevDriver member @vfat.
>> >> 
>> >> Unlike your patches, this does not require finding a "somewhere" that
>> >> provides information on "something".  You simply tack "deprecated" right
>> >> onto "something".
>> >> 
>> >> Your patches provide more information, however: human-readable messages.
>> >
>> > It also includes a machine-friendly suggested alternative (which
>> > I think is even more important that the human-readable message).
>> 
>> I agree we should point to a preferred replacement whenever we deprecate
>> something.
>> 
>> We have to do it in documentation.  And we generally do, in
>> qemu-deprecated.texi.
>> 
>> How useful would doing it in QMP as well be?  Depends on what management
>> applications can do with the additional information.
>
> I expect it to be useful for things that have obvious
> replacements, like old machine type or CPU model versions.

I doubt a management application should apply suggested replacements
automatically, and I doubt libvirt would.  Not even when QEMU developers
deem them "obvious".

>> > We could extend QAPI introspection to return that if necessary,
>> > right?
>> 
>> I'm confident we can come up with *something*.  It might kill the neat
>> and simple "use QAPI features to communicate deprecation" idea, though.
>
> If something is important enough to be communicated through
> stderr, it's important enough to be communicated through QMP.

Mostly.  Differences are due to the different consumers.

stderr is primarily for human users.  We print stuff useful to human
users.

QMP is primarily for machines, secondarily for the humans building these
machines.  We send stuff useful to the machines themselves, and stuff
the machines can use to be more useful for their users (which may be
machines or humans).  We can also send stuff to help the humans building
the machines.

In any case, the information we provide is limited by the cost to
provide it.

> Is that enough reason to provide something more complex?

We need to consider cost / benefit.

On benefit, I'd like to know what libvirt would do with the additional
information beyond logging it.

Is the additional information you propose to provide static or dynamic?

By "static", I mean each occurence of a feature in the QAPI schema is
tied to one fixed instance of "additional information".

> Do we need QAPI features to be just strings?  Can't they be a
> more complex type, like a QAPI alternate?

Adds complexity.

We currently imagine QAPI features enum-like, i.e. a list of strings,
optionally with conditions.  The conditions are evaluated at QAPI
generation time, and not visible in introspection.

This is probably the stupidest solution that could possibly work.  The
structure of features is trivial.

More expressive solutions include:

* List of 'any'.  Feels like a bad idea, because it's completely
  unstructured.

* List of some 'QapiFeatures' object type.  Lets us expose the variable
  structure of features in introspection.

* List of some 'QapiFeatures' alternate type.  Like the previous, but
  permits coding simple feature flags as strings.

All of the more expressive solutions I listed treat the additional
information as dynamic.  Overly general in case the information is
actually always static.

>> >> Food for thought :)
>> >> 
>> >> 
>> >> [*] Hiding in
>> >> Subject: [PATCH 0/4] file-posix: Add dynamic-auto-read-only QAPI feature
>> >> Date: Mon,  8 Apr 2019 16:35:39 +0200
>> >> Message-Id: <20190408143543.3982-1-kwolf@redhat.com>


[1] Special case of "merge additional information into QAPI/QMP
introspection at run-time, relegated to a footnote, because I think it's
of mostly historical interest now.

Qdev was designed to be declarative, but the declarations always got
collected only at run-time.  It has since been rebased onto imperative
QOM.  Since you can mix declarative qdev and imperative QOM, the
combination is effectively imperative.

We could collect the declarative parts of qdev at compile-time somehow,
and fold them into QAPI introspection.  That's the "Collect drives at
compile time" item from the slide I quoted.  I think this idea has
become pretty much worthless, since it would cover device_add only in
part, and object-add not at all.
Daniel P. Berrangé May 9, 2019, 9:14 a.m. UTC | #14
On Thu, May 09, 2019 at 10:31:46AM +0200, Markus Armbruster wrote:
> We've wandered into the QAPI vs. QOM swamp.  Cc: Paolo.
> 
> Eduardo Habkost <ehabkost@redhat.com> writes:
> 
> > On Wed, May 08, 2019 at 11:16:50AM +0200, Markus Armbruster wrote:
> >> Eduardo Habkost <ehabkost@redhat.com> writes:
> >> 
> >> > On Tue, May 07, 2019 at 07:07:04AM +0200, Markus Armbruster wrote:
> >> >> Eduardo Habkost <ehabkost@redhat.com> writes:
> >> >> 
> >> >> > This series adds machine type deprecation information to the
> >> >> > output of the `query-machines` QMP command.  With this, libvirt
> >> >> > and management software will be able to show this information to
> >> >> > users and/or suggest changes to VM configuration to avoid
> >> >> > deprecated machine types.
> >> >> 
> >> >> This overlaps with something I want to try, namely using Kevin's
> >> >> proposed QAPI feature flags for deprecation markings.  Let's compare the
> >> >> two.
> >> >> 
> >> >> To mark something as deprecated with your patches, you add a
> >> >> @support-status member somewhere, where "somewhere" is related to
> >> >> "something" by "provides information on".
> >> >> 
> >> >> Example: MachineInfo (returned by query-machines) provides information
> >> >> on possible values of -machine parameter type.  If -machine was
> >> >> QAPIfied, it would provide information on possible values of a QAPI
> >> >> object type's member.  The type might be anonymous.  The member should
> >> >> be an enum (we currently use 'str' in MachineInfo).
> >> >
> >> > QAPIfying -machine, -cpu, and -device would be wonderful.
> >> >
> >> >> 
> >> >> Example: say we want to deprecate block driver "vfat",
> >> >> i.e. BlockdevDriver member @vfat.  Type BlockdevDriver is used in
> >> >> multiple places; let's ignore all but BlockdevOptions.  We need to add
> >> >> @support-status to something that provides information on
> >> >> BlockdevDriver, or maybe on BlockdevOptions.  There is no ad hoc query
> >> >> providing information on either of the two, because QAPI/QMP
> >> >> introspection has been sufficient.  What now?
> >> >> 
> >> >> Can we add deprecation information to (general) QAPI/QMP introspection
> >> >
> >> > Yes, we can.  I think it's a good idea.  But:
> >> >
> >> >> instead of ad hoc queries?
> >> >
> >> > I'm not sure about the "instead of" part.  I don't want perfect
> >> > to be the enemy of done, and I don't want QAPIfication of
> >> > -machine to be a requirement to start reporting machine type
> >> > deprecation information.
> >> 
> >> Valid point.  Still, I believe we should at least try to predict how the
> >> pieces we create now would fit with the pieces we plan to create later
> >> on.
> >
> > Sure.
> >
> >> 
> >> Note that full QAPIfication of -machine isn't necessary to make QAPI
> >> feature "deprecated" work for machine types.  Turning MachineInfo member
> >> @name into an enum, so we can tack "deprecated" onto its values, would
> >> suffice.
> >> 
> >> Such a QAPIfication of machine types is still hard: QOM types are
> >> defined at compile time just like the QAPI schema, but their definition
> >> is distributed, and collected into one place only at run time.  I
> >> discussed this on slide 39 of my "QEMU interface introspection: From
> >> hacks to solutions" talk (KVM Form 2015).  Just for device_add, but it's
> >> just a special case of QOM.  Choices listed there:
> >> 
> >> * Collect drivers at compile time? Hard...
> >> * Make QAPI schema dynamic? Hard...
> >> * Forgo driver-specific arguments in schema?
> >>   Defeats introspection...
> >> 
> >> I'd like to add to the last item:
> >> 
> >>   Provide QOM introspection on par with QAPI schema introspection
> >> 
> >> The QOM introspection we have (qom-list-types etc. is not on par.
> >
> > Agreed, but do we really want to do it?  We have been avoiding
> > exposing QOM internals to the outside on purpose.  I believe
> > there are at least two reasons for that:
> >
> > 1) Not every QOM type/property is supposed to be visible to the
> >    outside
> 
> True.
> 
> However, the parts of QOM exposed via device_add and object-add are
> definitely part of the stable external interface (unless explicitly
> marked unstable).
> 
> >            (and nobody really knows what's the full set of
> >    supported external QOM interfaces);
> 
> Also true.  And terrible.
> 
> > 2) QAPI is our preferred interface interface specification/introspection
> >    mechanism.
> 
> When preferences and requirements collide, preferences tend to get run
> over.
> 
> The QAPI schema is *declarative*: the schema declares QAPI objects and
> properties.  We generate C from the schema, which we then compile and
> link into QEMU.
> 
> QOM is by design *imperative*: we execute compiled C at QEMU run-time to
> define QOM objects and properties.  Maximizes flexibility.  See also
> Turing tarpit.
> 
> No matter how much we'd prefer to use QAPI to specify external
> interfaces to QOM, we can't without making QAPI much more dynamic or QOM
> much more static.  Either is hard.  Quite possibly infeasible.
> 
> We could try to extend QAPI/QMP introspection to somehow merge in
> additional information at run-time[1].  Could be regarded as a limited
> way to make QAPI more dynamic.  This is in the "vague idea, not sure
> it's feasible" stage.
> 
> We could try to rearchitect QOM so that you can optionally specify QOM
> stuff in the QAPI schema, then require that for QOM stuff that's part of
> a stable external interface.  Same "vague idea" stage, feasibility even
> more doubtful.

I think it is feasible to use QAPI to declare the QOM object type,
QOM parent type, list of QOM interfaces and QOM properties in a
declarative manner. From that generate all the tedious boilerplate
code. The dev then just has to provide the "interesting" code for
the object.

I'd really like to do a PoC of this but never have free time :-(
Perhaps an interesting task for someone who wants to delve into
some new work...


> >> >> Example: to deprecate block driver "vfat", add feature "deprecated" to
> >> >> BlockdevDriver member @vfat.
> >> >> 
> >> >> Unlike your patches, this does not require finding a "somewhere" that
> >> >> provides information on "something".  You simply tack "deprecated" right
> >> >> onto "something".
> >> >> 
> >> >> Your patches provide more information, however: human-readable messages.
> >> >
> >> > It also includes a machine-friendly suggested alternative (which
> >> > I think is even more important that the human-readable message).
> >> 
> >> I agree we should point to a preferred replacement whenever we deprecate
> >> something.
> >> 
> >> We have to do it in documentation.  And we generally do, in
> >> qemu-deprecated.texi.
> >> 
> >> How useful would doing it in QMP as well be?  Depends on what management
> >> applications can do with the additional information.
> >
> > I expect it to be useful for things that have obvious
> > replacements, like old machine type or CPU model versions.
> 
> I doubt a management application should apply suggested replacements
> automatically, and I doubt libvirt would.  Not even when QEMU developers
> deem them "obvious".

We certainly won't apply the suggested replacement as in many cases
it is not going to be a functionally equivalent drop-in.

If QEMU logs it to stderr, it will end up in the per-VM log file
libvirt has under /var/log/libvirt/qemu/$GUESTNAME.log.  If QEMU
doesn't log it to stderr, then libvirt would just write it to
that same log file itself.

If libvirt gains some API or event for notifying apps of deprecation
we might bubble it up to the mgmt app that way.

I still feel it is useful to have the suggested replacement in the
logs, rather  than only leaving it in qemu-deprecated.texi.  This
way the info is immediately visible to both app developers and any
support person dealing with bugs.

If the app dev see the suggested replacement upfront they're more
likely to make an immediate decision to update their code if the
suggestion is trivial. If they need to go find the QEMU docs to
lookup what action is required I feel they'll more likely just
put the item on their long todo list where it will languish.

Regards,
Daniel
Eduardo Habkost May 9, 2019, 3:52 p.m. UTC | #15
On Thu, May 09, 2019 at 10:14:52AM +0100, Daniel P. Berrangé wrote:
> On Thu, May 09, 2019 at 10:31:46AM +0200, Markus Armbruster wrote:
> > We've wandered into the QAPI vs. QOM swamp.  Cc: Paolo.
> > 
> > Eduardo Habkost <ehabkost@redhat.com> writes:
> > 
> > > On Wed, May 08, 2019 at 11:16:50AM +0200, Markus Armbruster wrote:
> > >> Eduardo Habkost <ehabkost@redhat.com> writes:
> > >> 
> > >> > On Tue, May 07, 2019 at 07:07:04AM +0200, Markus Armbruster wrote:
> > >> >> Eduardo Habkost <ehabkost@redhat.com> writes:
> > >> >> 
> > >> >> > This series adds machine type deprecation information to the
> > >> >> > output of the `query-machines` QMP command.  With this, libvirt
> > >> >> > and management software will be able to show this information to
> > >> >> > users and/or suggest changes to VM configuration to avoid
> > >> >> > deprecated machine types.
> > >> >> 
> > >> >> This overlaps with something I want to try, namely using Kevin's
> > >> >> proposed QAPI feature flags for deprecation markings.  Let's compare the
> > >> >> two.
> > >> >> 
> > >> >> To mark something as deprecated with your patches, you add a
> > >> >> @support-status member somewhere, where "somewhere" is related to
> > >> >> "something" by "provides information on".
> > >> >> 
> > >> >> Example: MachineInfo (returned by query-machines) provides information
> > >> >> on possible values of -machine parameter type.  If -machine was
> > >> >> QAPIfied, it would provide information on possible values of a QAPI
> > >> >> object type's member.  The type might be anonymous.  The member should
> > >> >> be an enum (we currently use 'str' in MachineInfo).
> > >> >
> > >> > QAPIfying -machine, -cpu, and -device would be wonderful.
> > >> >
> > >> >> 
> > >> >> Example: say we want to deprecate block driver "vfat",
> > >> >> i.e. BlockdevDriver member @vfat.  Type BlockdevDriver is used in
> > >> >> multiple places; let's ignore all but BlockdevOptions.  We need to add
> > >> >> @support-status to something that provides information on
> > >> >> BlockdevDriver, or maybe on BlockdevOptions.  There is no ad hoc query
> > >> >> providing information on either of the two, because QAPI/QMP
> > >> >> introspection has been sufficient.  What now?
> > >> >> 
> > >> >> Can we add deprecation information to (general) QAPI/QMP introspection
> > >> >
> > >> > Yes, we can.  I think it's a good idea.  But:
> > >> >
> > >> >> instead of ad hoc queries?
> > >> >
> > >> > I'm not sure about the "instead of" part.  I don't want perfect
> > >> > to be the enemy of done, and I don't want QAPIfication of
> > >> > -machine to be a requirement to start reporting machine type
> > >> > deprecation information.
> > >> 
> > >> Valid point.  Still, I believe we should at least try to predict how the
> > >> pieces we create now would fit with the pieces we plan to create later
> > >> on.
> > >
> > > Sure.
> > >
> > >> 
> > >> Note that full QAPIfication of -machine isn't necessary to make QAPI
> > >> feature "deprecated" work for machine types.  Turning MachineInfo member
> > >> @name into an enum, so we can tack "deprecated" onto its values, would
> > >> suffice.
> > >> 
> > >> Such a QAPIfication of machine types is still hard: QOM types are
> > >> defined at compile time just like the QAPI schema, but their definition
> > >> is distributed, and collected into one place only at run time.  I
> > >> discussed this on slide 39 of my "QEMU interface introspection: From
> > >> hacks to solutions" talk (KVM Form 2015).  Just for device_add, but it's
> > >> just a special case of QOM.  Choices listed there:
> > >> 
> > >> * Collect drivers at compile time? Hard...
> > >> * Make QAPI schema dynamic? Hard...
> > >> * Forgo driver-specific arguments in schema?
> > >>   Defeats introspection...
> > >> 
> > >> I'd like to add to the last item:
> > >> 
> > >>   Provide QOM introspection on par with QAPI schema introspection
> > >> 
> > >> The QOM introspection we have (qom-list-types etc. is not on par.
> > >
> > > Agreed, but do we really want to do it?  We have been avoiding
> > > exposing QOM internals to the outside on purpose.  I believe
> > > there are at least two reasons for that:
> > >
> > > 1) Not every QOM type/property is supposed to be visible to the
> > >    outside
> > 
> > True.
> > 
> > However, the parts of QOM exposed via device_add and object-add are
> > definitely part of the stable external interface (unless explicitly
> > marked unstable).
> > 
> > >            (and nobody really knows what's the full set of
> > >    supported external QOM interfaces);
> > 
> > Also true.  And terrible.
> > 
> > > 2) QAPI is our preferred interface interface specification/introspection
> > >    mechanism.
> > 
> > When preferences and requirements collide, preferences tend to get run
> > over.
> > 
> > The QAPI schema is *declarative*: the schema declares QAPI objects and
> > properties.  We generate C from the schema, which we then compile and
> > link into QEMU.
> > 
> > QOM is by design *imperative*: we execute compiled C at QEMU run-time to
> > define QOM objects and properties.  Maximizes flexibility.  See also
> > Turing tarpit.
> > 
> > No matter how much we'd prefer to use QAPI to specify external
> > interfaces to QOM, we can't without making QAPI much more dynamic or QOM
> > much more static.  Either is hard.  Quite possibly infeasible.
> > 
> > We could try to extend QAPI/QMP introspection to somehow merge in
> > additional information at run-time[1].  Could be regarded as a limited
> > way to make QAPI more dynamic.  This is in the "vague idea, not sure
> > it's feasible" stage.
> > 
> > We could try to rearchitect QOM so that you can optionally specify QOM
> > stuff in the QAPI schema, then require that for QOM stuff that's part of
> > a stable external interface.  Same "vague idea" stage, feasibility even
> > more doubtful.
> 
> I think it is feasible to use QAPI to declare the QOM object type,
> QOM parent type, list of QOM interfaces and QOM properties in a
> declarative manner. From that generate all the tedious boilerplate
> code. The dev then just has to provide the "interesting" code for
> the object.
> 
> I'd really like to do a PoC of this but never have free time :-(
> Perhaps an interesting task for someone who wants to delve into
> some new work...
> 
> 
> > >> >> Example: to deprecate block driver "vfat", add feature "deprecated" to
> > >> >> BlockdevDriver member @vfat.
> > >> >> 
> > >> >> Unlike your patches, this does not require finding a "somewhere" that
> > >> >> provides information on "something".  You simply tack "deprecated" right
> > >> >> onto "something".
> > >> >> 
> > >> >> Your patches provide more information, however: human-readable messages.
> > >> >
> > >> > It also includes a machine-friendly suggested alternative (which
> > >> > I think is even more important that the human-readable message).
> > >> 
> > >> I agree we should point to a preferred replacement whenever we deprecate
> > >> something.
> > >> 
> > >> We have to do it in documentation.  And we generally do, in
> > >> qemu-deprecated.texi.
> > >> 
> > >> How useful would doing it in QMP as well be?  Depends on what management
> > >> applications can do with the additional information.
> > >
> > > I expect it to be useful for things that have obvious
> > > replacements, like old machine type or CPU model versions.
> > 
> > I doubt a management application should apply suggested replacements
> > automatically, and I doubt libvirt would.  Not even when QEMU developers
> > deem them "obvious".
> 
> We certainly won't apply the suggested replacement as in many cases
> it is not going to be a functionally equivalent drop-in.

Who's "we"?

> 
> If QEMU logs it to stderr, it will end up in the per-VM log file
> libvirt has under /var/log/libvirt/qemu/$GUESTNAME.log.  If QEMU
> doesn't log it to stderr, then libvirt would just write it to
> that same log file itself.
> 
> If libvirt gains some API or event for notifying apps of deprecation
> we might bubble it up to the mgmt app that way.
> 
> I still feel it is useful to have the suggested replacement in the
> logs, rather  than only leaving it in qemu-deprecated.texi.  This
> way the info is immediately visible to both app developers and any
> support person dealing with bugs.
> 
> If the app dev see the suggested replacement upfront they're more
> likely to make an immediate decision to update their code if the
> suggestion is trivial. If they need to go find the QEMU docs to
> lookup what action is required I feel they'll more likely just
> put the item on their long todo list where it will languish.

Agreed.  However, note that the audience for deprecation
information is not just developers and support people.  End users
need to know when they are relying on a deprecated feature, and
applications should make it as easy as possible for them to
update their configurations.

I'm not suggesting the alternative would be applied
automatically.  But having the alternative available in a
machine-friendly way may be the difference between a unhelpful UI
that just tells the user there's some problem but can't give a
solution, and one that can really assist the user to fix the
problem.
Daniel P. Berrangé May 9, 2019, 4:08 p.m. UTC | #16
On Thu, May 09, 2019 at 12:52:47PM -0300, Eduardo Habkost wrote:
> On Thu, May 09, 2019 at 10:14:52AM +0100, Daniel P. Berrangé wrote:
> > On Thu, May 09, 2019 at 10:31:46AM +0200, Markus Armbruster wrote:
> > > We've wandered into the QAPI vs. QOM swamp.  Cc: Paolo.
> > > 
> > > Eduardo Habkost <ehabkost@redhat.com> writes:
> > > 
> > > > On Wed, May 08, 2019 at 11:16:50AM +0200, Markus Armbruster wrote:
> > > >> Eduardo Habkost <ehabkost@redhat.com> writes:
> > > >> 
> > > >> > On Tue, May 07, 2019 at 07:07:04AM +0200, Markus Armbruster wrote:
> > > >> >> Eduardo Habkost <ehabkost@redhat.com> writes:
> > > >> >> 
> > > >> >> > This series adds machine type deprecation information to the
> > > >> >> > output of the `query-machines` QMP command.  With this, libvirt
> > > >> >> > and management software will be able to show this information to
> > > >> >> > users and/or suggest changes to VM configuration to avoid
> > > >> >> > deprecated machine types.
> > > >> >> 
> > > >> >> This overlaps with something I want to try, namely using Kevin's
> > > >> >> proposed QAPI feature flags for deprecation markings.  Let's compare the
> > > >> >> two.
> > > >> >> 
> > > >> >> To mark something as deprecated with your patches, you add a
> > > >> >> @support-status member somewhere, where "somewhere" is related to
> > > >> >> "something" by "provides information on".
> > > >> >> 
> > > >> >> Example: MachineInfo (returned by query-machines) provides information
> > > >> >> on possible values of -machine parameter type.  If -machine was
> > > >> >> QAPIfied, it would provide information on possible values of a QAPI
> > > >> >> object type's member.  The type might be anonymous.  The member should
> > > >> >> be an enum (we currently use 'str' in MachineInfo).
> > > >> >
> > > >> > QAPIfying -machine, -cpu, and -device would be wonderful.
> > > >> >
> > > >> >> 
> > > >> >> Example: say we want to deprecate block driver "vfat",
> > > >> >> i.e. BlockdevDriver member @vfat.  Type BlockdevDriver is used in
> > > >> >> multiple places; let's ignore all but BlockdevOptions.  We need to add
> > > >> >> @support-status to something that provides information on
> > > >> >> BlockdevDriver, or maybe on BlockdevOptions.  There is no ad hoc query
> > > >> >> providing information on either of the two, because QAPI/QMP
> > > >> >> introspection has been sufficient.  What now?
> > > >> >> 
> > > >> >> Can we add deprecation information to (general) QAPI/QMP introspection
> > > >> >
> > > >> > Yes, we can.  I think it's a good idea.  But:
> > > >> >
> > > >> >> instead of ad hoc queries?
> > > >> >
> > > >> > I'm not sure about the "instead of" part.  I don't want perfect
> > > >> > to be the enemy of done, and I don't want QAPIfication of
> > > >> > -machine to be a requirement to start reporting machine type
> > > >> > deprecation information.
> > > >> 
> > > >> Valid point.  Still, I believe we should at least try to predict how the
> > > >> pieces we create now would fit with the pieces we plan to create later
> > > >> on.
> > > >
> > > > Sure.
> > > >
> > > >> 
> > > >> Note that full QAPIfication of -machine isn't necessary to make QAPI
> > > >> feature "deprecated" work for machine types.  Turning MachineInfo member
> > > >> @name into an enum, so we can tack "deprecated" onto its values, would
> > > >> suffice.
> > > >> 
> > > >> Such a QAPIfication of machine types is still hard: QOM types are
> > > >> defined at compile time just like the QAPI schema, but their definition
> > > >> is distributed, and collected into one place only at run time.  I
> > > >> discussed this on slide 39 of my "QEMU interface introspection: From
> > > >> hacks to solutions" talk (KVM Form 2015).  Just for device_add, but it's
> > > >> just a special case of QOM.  Choices listed there:
> > > >> 
> > > >> * Collect drivers at compile time? Hard...
> > > >> * Make QAPI schema dynamic? Hard...
> > > >> * Forgo driver-specific arguments in schema?
> > > >>   Defeats introspection...
> > > >> 
> > > >> I'd like to add to the last item:
> > > >> 
> > > >>   Provide QOM introspection on par with QAPI schema introspection
> > > >> 
> > > >> The QOM introspection we have (qom-list-types etc. is not on par.
> > > >
> > > > Agreed, but do we really want to do it?  We have been avoiding
> > > > exposing QOM internals to the outside on purpose.  I believe
> > > > there are at least two reasons for that:
> > > >
> > > > 1) Not every QOM type/property is supposed to be visible to the
> > > >    outside
> > > 
> > > True.
> > > 
> > > However, the parts of QOM exposed via device_add and object-add are
> > > definitely part of the stable external interface (unless explicitly
> > > marked unstable).
> > > 
> > > >            (and nobody really knows what's the full set of
> > > >    supported external QOM interfaces);
> > > 
> > > Also true.  And terrible.
> > > 
> > > > 2) QAPI is our preferred interface interface specification/introspection
> > > >    mechanism.
> > > 
> > > When preferences and requirements collide, preferences tend to get run
> > > over.
> > > 
> > > The QAPI schema is *declarative*: the schema declares QAPI objects and
> > > properties.  We generate C from the schema, which we then compile and
> > > link into QEMU.
> > > 
> > > QOM is by design *imperative*: we execute compiled C at QEMU run-time to
> > > define QOM objects and properties.  Maximizes flexibility.  See also
> > > Turing tarpit.
> > > 
> > > No matter how much we'd prefer to use QAPI to specify external
> > > interfaces to QOM, we can't without making QAPI much more dynamic or QOM
> > > much more static.  Either is hard.  Quite possibly infeasible.
> > > 
> > > We could try to extend QAPI/QMP introspection to somehow merge in
> > > additional information at run-time[1].  Could be regarded as a limited
> > > way to make QAPI more dynamic.  This is in the "vague idea, not sure
> > > it's feasible" stage.
> > > 
> > > We could try to rearchitect QOM so that you can optionally specify QOM
> > > stuff in the QAPI schema, then require that for QOM stuff that's part of
> > > a stable external interface.  Same "vague idea" stage, feasibility even
> > > more doubtful.
> > 
> > I think it is feasible to use QAPI to declare the QOM object type,
> > QOM parent type, list of QOM interfaces and QOM properties in a
> > declarative manner. From that generate all the tedious boilerplate
> > code. The dev then just has to provide the "interesting" code for
> > the object.
> > 
> > I'd really like to do a PoC of this but never have free time :-(
> > Perhaps an interesting task for someone who wants to delve into
> > some new work...
> > 
> > 
> > > >> >> Example: to deprecate block driver "vfat", add feature "deprecated" to
> > > >> >> BlockdevDriver member @vfat.
> > > >> >> 
> > > >> >> Unlike your patches, this does not require finding a "somewhere" that
> > > >> >> provides information on "something".  You simply tack "deprecated" right
> > > >> >> onto "something".
> > > >> >> 
> > > >> >> Your patches provide more information, however: human-readable messages.
> > > >> >
> > > >> > It also includes a machine-friendly suggested alternative (which
> > > >> > I think is even more important that the human-readable message).
> > > >> 
> > > >> I agree we should point to a preferred replacement whenever we deprecate
> > > >> something.
> > > >> 
> > > >> We have to do it in documentation.  And we generally do, in
> > > >> qemu-deprecated.texi.
> > > >> 
> > > >> How useful would doing it in QMP as well be?  Depends on what management
> > > >> applications can do with the additional information.
> > > >
> > > > I expect it to be useful for things that have obvious
> > > > replacements, like old machine type or CPU model versions.
> > > 
> > > I doubt a management application should apply suggested replacements
> > > automatically, and I doubt libvirt would.  Not even when QEMU developers
> > > deem them "obvious".
> > 
> > We certainly won't apply the suggested replacement as in many cases
> > it is not going to be a functionally equivalent drop-in.
> 
> Who's "we"?

I was refering to libvirt here.

> > If QEMU logs it to stderr, it will end up in the per-VM log file
> > libvirt has under /var/log/libvirt/qemu/$GUESTNAME.log.  If QEMU
> > doesn't log it to stderr, then libvirt would just write it to
> > that same log file itself.
> > 
> > If libvirt gains some API or event for notifying apps of deprecation
> > we might bubble it up to the mgmt app that way.
> > 
> > I still feel it is useful to have the suggested replacement in the
> > logs, rather  than only leaving it in qemu-deprecated.texi.  This
> > way the info is immediately visible to both app developers and any
> > support person dealing with bugs.
> > 
> > If the app dev see the suggested replacement upfront they're more
> > likely to make an immediate decision to update their code if the
> > suggestion is trivial. If they need to go find the QEMU docs to
> > lookup what action is required I feel they'll more likely just
> > put the item on their long todo list where it will languish.
> 
> Agreed.  However, note that the audience for deprecation
> information is not just developers and support people.  End users
> need to know when they are relying on a deprecated feature, and
> applications should make it as easy as possible for them to
> update their configurations.
> 
> I'm not suggesting the alternative would be applied
> automatically.  But having the alternative available in a
> machine-friendly way may be the difference between a unhelpful UI
> that just tells the user there's some problem but can't give a
> solution, and one that can really assist the user to fix the
> problem.

For some aspects of QEMU it might be possible, but considering the
broader set of things which can be deprecated, I don't think it is
possible to expose a machine consumable "suggestion".

Consider the deprecation of the ACL feature. We deprecated monitor
commands "acl_add", "acl_policy", "acl_reset", etc. The suggested
replacement is to use one of the many possible QAuthZ types combined
with the -object arg. Even if we invented some way to express this
in the schema, I don't think any app would usefully consume it.

Regards,
Daniel
Eduardo Habkost May 9, 2019, 5:44 p.m. UTC | #17
On Thu, May 09, 2019 at 05:08:11PM +0100, Daniel P. Berrangé wrote:
> On Thu, May 09, 2019 at 12:52:47PM -0300, Eduardo Habkost wrote:
> > On Thu, May 09, 2019 at 10:14:52AM +0100, Daniel P. Berrangé wrote:
> > > On Thu, May 09, 2019 at 10:31:46AM +0200, Markus Armbruster wrote:
> > > > We've wandered into the QAPI vs. QOM swamp.  Cc: Paolo.
> > > > 
> > > > Eduardo Habkost <ehabkost@redhat.com> writes:
> > > > 
> > > > > On Wed, May 08, 2019 at 11:16:50AM +0200, Markus Armbruster wrote:
> > > > >> Eduardo Habkost <ehabkost@redhat.com> writes:
> > > > >> 
> > > > >> > On Tue, May 07, 2019 at 07:07:04AM +0200, Markus Armbruster wrote:
> > > > >> >> Eduardo Habkost <ehabkost@redhat.com> writes:
> > > > >> >> 
> > > > >> >> > This series adds machine type deprecation information to the
> > > > >> >> > output of the `query-machines` QMP command.  With this, libvirt
> > > > >> >> > and management software will be able to show this information to
> > > > >> >> > users and/or suggest changes to VM configuration to avoid
> > > > >> >> > deprecated machine types.
> > > > >> >> 
> > > > >> >> This overlaps with something I want to try, namely using Kevin's
> > > > >> >> proposed QAPI feature flags for deprecation markings.  Let's compare the
> > > > >> >> two.
> > > > >> >> 
> > > > >> >> To mark something as deprecated with your patches, you add a
> > > > >> >> @support-status member somewhere, where "somewhere" is related to
> > > > >> >> "something" by "provides information on".
> > > > >> >> 
> > > > >> >> Example: MachineInfo (returned by query-machines) provides information
> > > > >> >> on possible values of -machine parameter type.  If -machine was
> > > > >> >> QAPIfied, it would provide information on possible values of a QAPI
> > > > >> >> object type's member.  The type might be anonymous.  The member should
> > > > >> >> be an enum (we currently use 'str' in MachineInfo).
> > > > >> >
> > > > >> > QAPIfying -machine, -cpu, and -device would be wonderful.
> > > > >> >
> > > > >> >> 
> > > > >> >> Example: say we want to deprecate block driver "vfat",
> > > > >> >> i.e. BlockdevDriver member @vfat.  Type BlockdevDriver is used in
> > > > >> >> multiple places; let's ignore all but BlockdevOptions.  We need to add
> > > > >> >> @support-status to something that provides information on
> > > > >> >> BlockdevDriver, or maybe on BlockdevOptions.  There is no ad hoc query
> > > > >> >> providing information on either of the two, because QAPI/QMP
> > > > >> >> introspection has been sufficient.  What now?
> > > > >> >> 
> > > > >> >> Can we add deprecation information to (general) QAPI/QMP introspection
> > > > >> >
> > > > >> > Yes, we can.  I think it's a good idea.  But:
> > > > >> >
> > > > >> >> instead of ad hoc queries?
> > > > >> >
> > > > >> > I'm not sure about the "instead of" part.  I don't want perfect
> > > > >> > to be the enemy of done, and I don't want QAPIfication of
> > > > >> > -machine to be a requirement to start reporting machine type
> > > > >> > deprecation information.
> > > > >> 
> > > > >> Valid point.  Still, I believe we should at least try to predict how the
> > > > >> pieces we create now would fit with the pieces we plan to create later
> > > > >> on.
> > > > >
> > > > > Sure.
> > > > >
> > > > >> 
> > > > >> Note that full QAPIfication of -machine isn't necessary to make QAPI
> > > > >> feature "deprecated" work for machine types.  Turning MachineInfo member
> > > > >> @name into an enum, so we can tack "deprecated" onto its values, would
> > > > >> suffice.
> > > > >> 
> > > > >> Such a QAPIfication of machine types is still hard: QOM types are
> > > > >> defined at compile time just like the QAPI schema, but their definition
> > > > >> is distributed, and collected into one place only at run time.  I
> > > > >> discussed this on slide 39 of my "QEMU interface introspection: From
> > > > >> hacks to solutions" talk (KVM Form 2015).  Just for device_add, but it's
> > > > >> just a special case of QOM.  Choices listed there:
> > > > >> 
> > > > >> * Collect drivers at compile time? Hard...
> > > > >> * Make QAPI schema dynamic? Hard...
> > > > >> * Forgo driver-specific arguments in schema?
> > > > >>   Defeats introspection...
> > > > >> 
> > > > >> I'd like to add to the last item:
> > > > >> 
> > > > >>   Provide QOM introspection on par with QAPI schema introspection
> > > > >> 
> > > > >> The QOM introspection we have (qom-list-types etc. is not on par.
> > > > >
> > > > > Agreed, but do we really want to do it?  We have been avoiding
> > > > > exposing QOM internals to the outside on purpose.  I believe
> > > > > there are at least two reasons for that:
> > > > >
> > > > > 1) Not every QOM type/property is supposed to be visible to the
> > > > >    outside
> > > > 
> > > > True.
> > > > 
> > > > However, the parts of QOM exposed via device_add and object-add are
> > > > definitely part of the stable external interface (unless explicitly
> > > > marked unstable).
> > > > 
> > > > >            (and nobody really knows what's the full set of
> > > > >    supported external QOM interfaces);
> > > > 
> > > > Also true.  And terrible.
> > > > 
> > > > > 2) QAPI is our preferred interface interface specification/introspection
> > > > >    mechanism.
> > > > 
> > > > When preferences and requirements collide, preferences tend to get run
> > > > over.
> > > > 
> > > > The QAPI schema is *declarative*: the schema declares QAPI objects and
> > > > properties.  We generate C from the schema, which we then compile and
> > > > link into QEMU.
> > > > 
> > > > QOM is by design *imperative*: we execute compiled C at QEMU run-time to
> > > > define QOM objects and properties.  Maximizes flexibility.  See also
> > > > Turing tarpit.
> > > > 
> > > > No matter how much we'd prefer to use QAPI to specify external
> > > > interfaces to QOM, we can't without making QAPI much more dynamic or QOM
> > > > much more static.  Either is hard.  Quite possibly infeasible.
> > > > 
> > > > We could try to extend QAPI/QMP introspection to somehow merge in
> > > > additional information at run-time[1].  Could be regarded as a limited
> > > > way to make QAPI more dynamic.  This is in the "vague idea, not sure
> > > > it's feasible" stage.
> > > > 
> > > > We could try to rearchitect QOM so that you can optionally specify QOM
> > > > stuff in the QAPI schema, then require that for QOM stuff that's part of
> > > > a stable external interface.  Same "vague idea" stage, feasibility even
> > > > more doubtful.
> > > 
> > > I think it is feasible to use QAPI to declare the QOM object type,
> > > QOM parent type, list of QOM interfaces and QOM properties in a
> > > declarative manner. From that generate all the tedious boilerplate
> > > code. The dev then just has to provide the "interesting" code for
> > > the object.
> > > 
> > > I'd really like to do a PoC of this but never have free time :-(
> > > Perhaps an interesting task for someone who wants to delve into
> > > some new work...
> > > 
> > > 
> > > > >> >> Example: to deprecate block driver "vfat", add feature "deprecated" to
> > > > >> >> BlockdevDriver member @vfat.
> > > > >> >> 
> > > > >> >> Unlike your patches, this does not require finding a "somewhere" that
> > > > >> >> provides information on "something".  You simply tack "deprecated" right
> > > > >> >> onto "something".
> > > > >> >> 
> > > > >> >> Your patches provide more information, however: human-readable messages.
> > > > >> >
> > > > >> > It also includes a machine-friendly suggested alternative (which
> > > > >> > I think is even more important that the human-readable message).
> > > > >> 
> > > > >> I agree we should point to a preferred replacement whenever we deprecate
> > > > >> something.
> > > > >> 
> > > > >> We have to do it in documentation.  And we generally do, in
> > > > >> qemu-deprecated.texi.
> > > > >> 
> > > > >> How useful would doing it in QMP as well be?  Depends on what management
> > > > >> applications can do with the additional information.
> > > > >
> > > > > I expect it to be useful for things that have obvious
> > > > > replacements, like old machine type or CPU model versions.
> > > > 
> > > > I doubt a management application should apply suggested replacements
> > > > automatically, and I doubt libvirt would.  Not even when QEMU developers
> > > > deem them "obvious".
> > > 
> > > We certainly won't apply the suggested replacement as in many cases
> > > it is not going to be a functionally equivalent drop-in.
> > 
> > Who's "we"?
> 
> I was refering to libvirt here.
> 
> > > If QEMU logs it to stderr, it will end up in the per-VM log file
> > > libvirt has under /var/log/libvirt/qemu/$GUESTNAME.log.  If QEMU
> > > doesn't log it to stderr, then libvirt would just write it to
> > > that same log file itself.
> > > 
> > > If libvirt gains some API or event for notifying apps of deprecation
> > > we might bubble it up to the mgmt app that way.
> > > 
> > > I still feel it is useful to have the suggested replacement in the
> > > logs, rather  than only leaving it in qemu-deprecated.texi.  This
> > > way the info is immediately visible to both app developers and any
> > > support person dealing with bugs.
> > > 
> > > If the app dev see the suggested replacement upfront they're more
> > > likely to make an immediate decision to update their code if the
> > > suggestion is trivial. If they need to go find the QEMU docs to
> > > lookup what action is required I feel they'll more likely just
> > > put the item on their long todo list where it will languish.
> > 
> > Agreed.  However, note that the audience for deprecation
> > information is not just developers and support people.  End users
> > need to know when they are relying on a deprecated feature, and
> > applications should make it as easy as possible for them to
> > update their configurations.
> > 
> > I'm not suggesting the alternative would be applied
> > automatically.  But having the alternative available in a
> > machine-friendly way may be the difference between a unhelpful UI
> > that just tells the user there's some problem but can't give a
> > solution, and one that can really assist the user to fix the
> > problem.
> 
> For some aspects of QEMU it might be possible, but considering the
> broader set of things which can be deprecated, I don't think it is
> possible to expose a machine consumable "suggestion".
> 
> Consider the deprecation of the ACL feature. We deprecated monitor
> commands "acl_add", "acl_policy", "acl_reset", etc. The suggested
> replacement is to use one of the many possible QAuthZ types combined
> with the -object arg. Even if we invented some way to express this
> in the schema, I don't think any app would usefully consume it.

No problem, we don't need to suggest a machine consumable
alternative for everything.

I'm thinking about features that are visible to the end user and
require user action to fix their configuration, like machine type
versions or CPU model versions.
Eduardo Habkost May 9, 2019, 6:19 p.m. UTC | #18
On Thu, May 09, 2019 at 10:31:46AM +0200, Markus Armbruster wrote:
> We've wandered into the QAPI vs. QOM swamp.  Cc: Paolo.
> 
> Eduardo Habkost <ehabkost@redhat.com> writes:
> 
> > On Wed, May 08, 2019 at 11:16:50AM +0200, Markus Armbruster wrote:
> >> Eduardo Habkost <ehabkost@redhat.com> writes:
> >> 
> >> > On Tue, May 07, 2019 at 07:07:04AM +0200, Markus Armbruster wrote:
> >> >> Eduardo Habkost <ehabkost@redhat.com> writes:
> >> >> 
> >> >> > This series adds machine type deprecation information to the
> >> >> > output of the `query-machines` QMP command.  With this, libvirt
> >> >> > and management software will be able to show this information to
> >> >> > users and/or suggest changes to VM configuration to avoid
> >> >> > deprecated machine types.
> >> >> 
> >> >> This overlaps with something I want to try, namely using Kevin's
> >> >> proposed QAPI feature flags for deprecation markings.  Let's compare the
> >> >> two.
> >> >> 
> >> >> To mark something as deprecated with your patches, you add a
> >> >> @support-status member somewhere, where "somewhere" is related to
> >> >> "something" by "provides information on".
> >> >> 
> >> >> Example: MachineInfo (returned by query-machines) provides information
> >> >> on possible values of -machine parameter type.  If -machine was
> >> >> QAPIfied, it would provide information on possible values of a QAPI
> >> >> object type's member.  The type might be anonymous.  The member should
> >> >> be an enum (we currently use 'str' in MachineInfo).
> >> >
> >> > QAPIfying -machine, -cpu, and -device would be wonderful.
> >> >
> >> >> 
> >> >> Example: say we want to deprecate block driver "vfat",
> >> >> i.e. BlockdevDriver member @vfat.  Type BlockdevDriver is used in
> >> >> multiple places; let's ignore all but BlockdevOptions.  We need to add
> >> >> @support-status to something that provides information on
> >> >> BlockdevDriver, or maybe on BlockdevOptions.  There is no ad hoc query
> >> >> providing information on either of the two, because QAPI/QMP
> >> >> introspection has been sufficient.  What now?
> >> >> 
> >> >> Can we add deprecation information to (general) QAPI/QMP introspection
> >> >
> >> > Yes, we can.  I think it's a good idea.  But:
> >> >
> >> >> instead of ad hoc queries?
> >> >
> >> > I'm not sure about the "instead of" part.  I don't want perfect
> >> > to be the enemy of done, and I don't want QAPIfication of
> >> > -machine to be a requirement to start reporting machine type
> >> > deprecation information.
> >> 
> >> Valid point.  Still, I believe we should at least try to predict how the
> >> pieces we create now would fit with the pieces we plan to create later
> >> on.
> >
> > Sure.
> >
> >> 
> >> Note that full QAPIfication of -machine isn't necessary to make QAPI
> >> feature "deprecated" work for machine types.  Turning MachineInfo member
> >> @name into an enum, so we can tack "deprecated" onto its values, would
> >> suffice.
> >> 
> >> Such a QAPIfication of machine types is still hard: QOM types are
> >> defined at compile time just like the QAPI schema, but their definition
> >> is distributed, and collected into one place only at run time.  I
> >> discussed this on slide 39 of my "QEMU interface introspection: From
> >> hacks to solutions" talk (KVM Form 2015).  Just for device_add, but it's
> >> just a special case of QOM.  Choices listed there:
> >> 
> >> * Collect drivers at compile time? Hard...
> >> * Make QAPI schema dynamic? Hard...
> >> * Forgo driver-specific arguments in schema?
> >>   Defeats introspection...
> >> 
> >> I'd like to add to the last item:
> >> 
> >>   Provide QOM introspection on par with QAPI schema introspection
> >> 
> >> The QOM introspection we have (qom-list-types etc. is not on par.
> >
> > Agreed, but do we really want to do it?  We have been avoiding
> > exposing QOM internals to the outside on purpose.  I believe
> > there are at least two reasons for that:
> >
> > 1) Not every QOM type/property is supposed to be visible to the
> >    outside
> 
> True.
> 
> However, the parts of QOM exposed via device_add and object-add are
> definitely part of the stable external interface (unless explicitly
> marked unstable).
> 
> >            (and nobody really knows what's the full set of
> >    supported external QOM interfaces);
> 
> Also true.  And terrible.
> 
> > 2) QAPI is our preferred interface interface specification/introspection
> >    mechanism.
> 
> When preferences and requirements collide, preferences tend to get run
> over.
> 
> The QAPI schema is *declarative*: the schema declares QAPI objects and
> properties.  We generate C from the schema, which we then compile and
> link into QEMU.
> 
> QOM is by design *imperative*: we execute compiled C at QEMU run-time to
> define QOM objects and properties.  Maximizes flexibility.  See also
> Turing tarpit.
> 
> No matter how much we'd prefer to use QAPI to specify external
> interfaces to QOM, we can't without making QAPI much more dynamic or QOM
> much more static.  Either is hard.  Quite possibly infeasible.
> 
> We could try to extend QAPI/QMP introspection to somehow merge in
> additional information at run-time[1].  Could be regarded as a limited
> way to make QAPI more dynamic.  This is in the "vague idea, not sure
> it's feasible" stage.
> 
> We could try to rearchitect QOM so that you can optionally specify QOM
> stuff in the QAPI schema, then require that for QOM stuff that's part of
> a stable external interface.  Same "vague idea" stage, feasibility even
> more doubtful.
> 
> If I remember correctly, Paolo suggested to abandon the idea to have
> query-qmp-schema cover device_add as infeasible.  Instead, cover it
> separately.
> 
> I think this separate introspection mechanism should be as similar to
> query-qmp-schema as practical.  It should cover not just device_add, but
> also object-add, and whatever other external interface to QOM we deem
> stable.

I think we are in agreement here.  We don't need to make the new
introspection mechanism part of query-qmp-schema, but I think it
should use the same abstractions.

> 
> Should there be more stable external interfaces that can't be defined in
> the QAPI schema, we might want to cover them in this second
> introspection mechanism, too.

Agreed.

> 
> >> Back to exposing machine type deprecation.
> >> 
> >> I'm doubtful your proposed solution can be applied widely.  It relies on
> >> adding @support-status to something that provides information on
> >> whatever is deprecated.  The initial use is with a something that is an
> >> ad hoc query, namely query-machines.  To use it, the management
> >> application needs to understand what query-machines' @support-status
> >> applies to.  Certainly feasible.  But I fear every use will be a special
> >> case.  Furthermore, a suitable ad hoc query need not exist.  What then?
> >> Create suitable ad hoc queries just for communicating deprecation?
> >> 
> >> Instead, I'd like us to think about a more genral solution.  Or perhaps
> >> two: one for properly QAPIfied stuff, and one for QOM.
> >
> > Should we really spend our time designing a second solution, if
> > we could build this on top of QAPI abstractions?  Making the QAPI
> > schema dynamic might be hard, but reinventing QAPI and
> > maintaining the two systems in parallel seems harder.
> 
> I'd love to see a practical way forward that keeps QAPI/QMP
> introspection the only game in town.
> 
> >> >> Kevin's proposed QAPI feature flags[*] extend the QAPI language so that
> >> >> struct types can optionally have a list of feature flags, which are
> >> >> strings.  Struct types suffice for his immediate needs.  I'd like to use
> >> >> feature flags to mark deprecation by tacking a "deprecated" feature onto
> >> >> whatever is deprecated.  This obviously needs feature support for
> >> >> everything we want to be able to deprecate: commands, and events, as
> >> >> well as members of enum and object types.
> >> >> 
> >> >> Example: to deprecate block driver "vfat", add feature "deprecated" to
> >> >> BlockdevDriver member @vfat.
> >> >> 
> >> >> Unlike your patches, this does not require finding a "somewhere" that
> >> >> provides information on "something".  You simply tack "deprecated" right
> >> >> onto "something".
> >> >> 
> >> >> Your patches provide more information, however: human-readable messages.
> >> >
> >> > It also includes a machine-friendly suggested alternative (which
> >> > I think is even more important that the human-readable message).
> >> 
> >> I agree we should point to a preferred replacement whenever we deprecate
> >> something.
> >> 
> >> We have to do it in documentation.  And we generally do, in
> >> qemu-deprecated.texi.
> >> 
> >> How useful would doing it in QMP as well be?  Depends on what management
> >> applications can do with the additional information.
> >
> > I expect it to be useful for things that have obvious
> > replacements, like old machine type or CPU model versions.
> 
> I doubt a management application should apply suggested replacements
> automatically, and I doubt libvirt would.  Not even when QEMU developers
> deem them "obvious".
> 
> >> > We could extend QAPI introspection to return that if necessary,
> >> > right?
> >> 
> >> I'm confident we can come up with *something*.  It might kill the neat
> >> and simple "use QAPI features to communicate deprecation" idea, though.
> >
> > If something is important enough to be communicated through
> > stderr, it's important enough to be communicated through QMP.
> 
> Mostly.  Differences are due to the different consumers.
> 
> stderr is primarily for human users.  We print stuff useful to human
> users.

We have users that don't have access to stderr.  They might have
access to log files, but log files are pretty bad user
interfaces.  If it's important for some set of human users, apps
using libvirt or QMP need access to that information so they can
show it to their human users too.

> 
> QMP is primarily for machines, secondarily for the humans building these
> machines.  We send stuff useful to the machines themselves, and stuff
> the machines can use to be more useful for their users (which may be
> machines or humans).  We can also send stuff to help the humans building
> the machines.
> 
> In any case, the information we provide is limited by the cost to
> provide it.

Absolutely.

> 
> > Is that enough reason to provide something more complex?
> 
> We need to consider cost / benefit.
> 
> On benefit, I'd like to know what libvirt would do with the additional
> information beyond logging it.

I'd say it should provide it to apps, otherwise this won't be
more useful than the existing log files.

> 
> Is the additional information you propose to provide static or dynamic?
> 
> By "static", I mean each occurence of a feature in the QAPI schema is
> tied to one fixed instance of "additional information".

I don't think I understand this description of "static".  I
expect the data to be fixed at build time, but I expect it to be
different in downstream distributions of QEMU.


> 
> > Do we need QAPI features to be just strings?  Can't they be a
> > more complex type, like a QAPI alternate?
> 
> Adds complexity.
> 
> We currently imagine QAPI features enum-like, i.e. a list of strings,
> optionally with conditions.  The conditions are evaluated at QAPI
> generation time, and not visible in introspection.
> 
> This is probably the stupidest solution that could possibly work.  The
> structure of features is trivial.
> 
> More expressive solutions include:
> 
> * List of 'any'.  Feels like a bad idea, because it's completely
>   unstructured.

Agreed.

> 
> * List of some 'QapiFeatures' object type.  Lets us expose the variable
>   structure of features in introspection.
> 
> * List of some 'QapiFeatures' alternate type.  Like the previous, but
>   permits coding simple feature flags as strings.
> 
> All of the more expressive solutions I listed treat the additional
> information as dynamic.  Overly general in case the information is
> actually always static.

The static vs. dynamic distinction is getting me confused.  Why
are the more expressive solutions more dynamic than "list of
strings"?  Can you give examples?

Also, why do we want to place all info inside the same "features"
attribute instead of just adding new fields to SchemaInfoObject?

i.e. why are these options:

  { "members" [ ... ],
    "features": [ "dynamic-read-only", "deprecated" ] }

  { "members" [ ... ],
    "features": [ "dynamic-read-only",
                  { "deprecated": true, "hint": "FOO" } ] }

better than these options:

  { "members" [ ... ],
    "features": [ "dynamic-read-only" ],
    "deprecated": true }

  { "members" [ ... ],
    "features": [ "dynamic-read-only" ],
    "deprecation-info": { "deprecated": true, "hint": "FOO" } }



> 
> >> >> Food for thought :)
> >> >> 
> >> >> 
> >> >> [*] Hiding in
> >> >> Subject: [PATCH 0/4] file-posix: Add dynamic-auto-read-only QAPI feature
> >> >> Date: Mon,  8 Apr 2019 16:35:39 +0200
> >> >> Message-Id: <20190408143543.3982-1-kwolf@redhat.com>
> 
> 
> [1] Special case of "merge additional information into QAPI/QMP
> introspection at run-time, relegated to a footnote, because I think it's
> of mostly historical interest now.
> 
> Qdev was designed to be declarative, but the declarations always got
> collected only at run-time.  It has since been rebased onto imperative
> QOM.  Since you can mix declarative qdev and imperative QOM, the
> combination is effectively imperative.
> 
> We could collect the declarative parts of qdev at compile-time somehow,
> and fold them into QAPI introspection.  That's the "Collect drives at
> compile time" item from the slide I quoted.  I think this idea has
> become pretty much worthless, since it would cover device_add only in
> part, and object-add not at all.