diff mbox series

[v4] introduce vfio-user protocol specification

Message ID 1600180157-74760-1-git-send-email-thanos.makatos@nutanix.com
State New
Headers show
Series [v4] introduce vfio-user protocol specification | expand

Commit Message

Thanos Makatos Sept. 15, 2020, 2:29 p.m. UTC
This patch introduces the vfio-user protocol specification (formerly
known as VFIO-over-socket), which is designed to allow devices to be
emulated outside QEMU, in a separate process. vfio-user reuses the
existing VFIO defines, structs and concepts.

It has been earlier discussed as an RFC in:
"RFC: use VFIO over a UNIX domain socket to implement device offloading"

Signed-off-by: John G Johnson <john.g.johnson@oracle.com>
Signed-off-by: Thanos Makatos <thanos.makatos@nutanix.com>

---

Changed since v1:
  * fix coding style issues
  * update MAINTAINERS for VFIO-over-socket
  * add vfio-over-socket to ToC

Changed since v2:
  * fix whitespace

Changed since v3:
  * rename protocol to vfio-user
  * add table of contents
  * fix Unicode problems
  * fix typos and various reStructuredText issues
  * various stylistic improvements
  * add backend program conventions
  * rewrite part of intro, drop QEMU-specific stuff
  * drop QEMU-specific paragraph about implementation
  * explain that passing of FDs isn't necessary
  * minor improvements in the VFIO section
  * various text substitutions for the sake of consistency
  * drop paragraph about client and server, already explained in intro
  * drop device ID
  * drop type from version
  * elaborate on request concurrency
  * convert some inessential paragraphs into notes
  * explain why some existing VFIO defines cannot be reused
  * explain how to make changes to the protocol
  * improve text of DMA map
  * reword comment about existing VFIO commands
  * add reference to Version section
  * reset device on disconnection
  * reword live migration section
  * replace sys/vfio.h with linux/vfio.h
  * drop reference to iovec
  * use argz the same way it is used in VFIO
  * add type field in header for clarity

Signed-off-by: John G Johnson <john.g.johnson@oracle.com>
Signed-off-by: Thanos Makatos <thanos.makatos@nutanix.com>
---
 MAINTAINERS              |    6 +
 docs/devel/index.rst     |    1 +
 docs/devel/vfio-user.rst | 1191 ++++++++++++++++++++++++++++++++++++++++++++++
 3 files changed, 1198 insertions(+)
 create mode 100644 docs/devel/vfio-user.rst

Comments

Stefan Hajnoczi Sept. 24, 2020, 8:21 a.m. UTC | #1
On Tue, Sep 15, 2020 at 07:29:17AM -0700, Thanos Makatos wrote:
> This patch introduces the vfio-user protocol specification (formerly
> known as VFIO-over-socket), which is designed to allow devices to be
> emulated outside QEMU, in a separate process. vfio-user reuses the
> existing VFIO defines, structs and concepts.
> 
> It has been earlier discussed as an RFC in:
> "RFC: use VFIO over a UNIX domain socket to implement device offloading"
> 
> Signed-off-by: John G Johnson <john.g.johnson@oracle.com>
> Signed-off-by: Thanos Makatos <thanos.makatos@nutanix.com>

The approach looks promising. It's hard to know what changes will be
required when this is implemented, so let's not worry about getting
every detail of the spec right.

Now that there is a spec to start from, the next step is patches
implementing --device vfio-user-pci,chardev=<chardev> in
hw/vfio-user/pci.c (mirroring hw/vfio/).

It should be accompanied by a test in tests/. PCI-level testing APIS for
BARs, configuration space, interrupts, etc are available in
tests/qtest/libqos/pci.h. The test case needs to include a vfio-user
device backend interact with QEMU's vfio-user-pci implementation.

I think this spec can be merged in docs/devel/ now and marked as
"subject to change (not a stable public interface)".

After the details have been proven and any necessary changes have been
made the spec can be promoted to docs/interop/ as a stable public
interface. This gives the freedom to make changes discovered when
figuring out issues like disconnect/reconnect, live migration, etc that
can be hard to get right without a working implementation.

Does this approach sound good?

Also please let us know who is working on what so additional people can
get involved in areas that need work!

Stefan
Michael S. Tsirkin Sept. 24, 2020, 9:24 a.m. UTC | #2
On Thu, Sep 24, 2020 at 09:21:32AM +0100, Stefan Hajnoczi wrote:
> On Tue, Sep 15, 2020 at 07:29:17AM -0700, Thanos Makatos wrote:
> > This patch introduces the vfio-user protocol specification (formerly
> > known as VFIO-over-socket), which is designed to allow devices to be
> > emulated outside QEMU, in a separate process. vfio-user reuses the
> > existing VFIO defines, structs and concepts.
> > 
> > It has been earlier discussed as an RFC in:
> > "RFC: use VFIO over a UNIX domain socket to implement device offloading"
> > 
> > Signed-off-by: John G Johnson <john.g.johnson@oracle.com>
> > Signed-off-by: Thanos Makatos <thanos.makatos@nutanix.com>
> 
> The approach looks promising. It's hard to know what changes will be
> required when this is implemented, so let's not worry about getting
> every detail of the spec right.
> 
> Now that there is a spec to start from, the next step is patches
> implementing --device vfio-user-pci,chardev=<chardev> in
> hw/vfio-user/pci.c (mirroring hw/vfio/).
> 
> It should be accompanied by a test in tests/. PCI-level testing APIS for
> BARs, configuration space, interrupts, etc are available in
> tests/qtest/libqos/pci.h. The test case needs to include a vfio-user
> device backend interact with QEMU's vfio-user-pci implementation.
> 
> I think this spec can be merged in docs/devel/ now and marked as
> "subject to change (not a stable public interface)".
> 
> After the details have been proven and any necessary changes have been
> made the spec can be promoted to docs/interop/ as a stable public
> interface. This gives the freedom to make changes discovered when
> figuring out issues like disconnect/reconnect, live migration, etc that
> can be hard to get right without a working implementation.
> 
> Does this approach sound good?
> 
> Also please let us know who is working on what so additional people can
> get involved in areas that need work!
> 
> Stefan

Problem we discovered with e.g. vhost is once you ship a management
interface, people start using it immediately and it does not matter
that you never promised stability.

So I feel a good first step would be to limit this to only allow known
in-tree devices, started/destroyed automatically by qemu when device is
created.  This way lots of reconnect etc issues go away, and we don't
commit to a stable protocol until we have a decent handle on how things
will work in production.
Thanos Makatos Sept. 28, 2020, 9:58 a.m. UTC | #3
> -----Original Message-----
> From: Stefan Hajnoczi <stefanha@redhat.com>
> Sent: 24 September 2020 09:22
> To: Thanos Makatos <thanos.makatos@nutanix.com>
> Cc: qemu-devel@nongnu.org; Michael S. Tsirkin <mst@redhat.com>;
> alex.williamson@redhat.com; benjamin.walker@intel.com;
> elena.ufimtseva@oracle.com; jag.raman@oracle.com; Swapnil Ingle
> <swapnil.ingle@nutanix.com>; james.r.harris@intel.com;
> konrad.wilk@oracle.com; Raphael Norwitz <raphael.norwitz@nutanix.com>;
> marcandre.lureau@redhat.com; Kanth.Ghatraju@oracle.com; Felipe
> Franciosi <felipe@nutanix.com>; tina.zhang@intel.com;
> changpeng.liu@intel.com; dgilbert@redhat.com;
> tomassetti.andrea@gmail.com; yuvalkashtan@gmail.com;
> ismael@linux.com; xiuchun.lu@intel.com; John G Johnson
> <john.g.johnson@oracle.com>
> Subject: Re: [PATCH v4] introduce vfio-user protocol specification
> 
> On Tue, Sep 15, 2020 at 07:29:17AM -0700, Thanos Makatos wrote:
> > This patch introduces the vfio-user protocol specification (formerly
> > known as VFIO-over-socket), which is designed to allow devices to be
> > emulated outside QEMU, in a separate process. vfio-user reuses the
> > existing VFIO defines, structs and concepts.
> >
> > It has been earlier discussed as an RFC in:
> > "RFC: use VFIO over a UNIX domain socket to implement device offloading"
> >
> > Signed-off-by: John G Johnson <john.g.johnson@oracle.com>
> > Signed-off-by: Thanos Makatos <thanos.makatos@nutanix.com>
> 
> The approach looks promising. It's hard to know what changes will be
> required when this is implemented, so let's not worry about getting
> every detail of the spec right.
> 
> Now that there is a spec to start from, the next step is patches
> implementing --device vfio-user-pci,chardev=<chardev> in
> hw/vfio-user/pci.c (mirroring hw/vfio/).
> 
> It should be accompanied by a test in tests/. PCI-level testing APIS for
> BARs, configuration space, interrupts, etc are available in
> tests/qtest/libqos/pci.h. The test case needs to include a vfio-user
> device backend interact with QEMU's vfio-user-pci implementation.

We plan to use a libmuser-based backend for testing. This, I suppose, will
make libmuser a dependency of QEMU (either as a submodule or as a library),
which for now can be disabled in the default configuration. Is this acceptable?

> 
> I think this spec can be merged in docs/devel/ now and marked as
> "subject to change (not a stable public interface

Great!

> 
> After the details have been proven and any necessary changes have been
> made the spec can be promoted to docs/interop/ as a stable public
> interface. This gives the freedom to make changes discovered when
> figuring out issues like disconnect/reconnect, live migration, etc that
> can be hard to get right without a working implementation.
> 
> Does this approach sound good?

Yes.

> 
> Also please let us know who is working on what so additional people can
> get involved in areas that need work!

Swapnil and I will be working on libmuser and the test in QEMU, John and
the mp-qemu folks will be working on the patches for implementing
--device vfio-user-pci.

> 
> Stefan
Stefan Hajnoczi Sept. 29, 2020, 10:37 a.m. UTC | #4
On Mon, Sep 28, 2020 at 09:58:37AM +0000, Thanos Makatos wrote:
> > It should be accompanied by a test in tests/. PCI-level testing APIS for
> > BARs, configuration space, interrupts, etc are available in
> > tests/qtest/libqos/pci.h. The test case needs to include a vfio-user
> > device backend interact with QEMU's vfio-user-pci implementation.
> 
> We plan to use a libmuser-based backend for testing. This, I suppose, will
> make libmuser a dependency of QEMU (either as a submodule or as a library),
> which for now can be disabled in the default configuration. Is this acceptable?

If there are no other dependencies and libmuser supports all host
operating systems that QEMU's -device vfio-user supports, then I think
it's a good idea to use libmuser for at least one in-tree test in QEMU.

> > Also please let us know who is working on what so additional people can
> > get involved in areas that need work!
> 
> Swapnil and I will be working on libmuser and the test in QEMU, John and
> the mp-qemu folks will be working on the patches for implementing
> --device vfio-user-pci.

Great!

John: Will mpqemu use libmuser to implement the remote PCI host
controller?

Stefan
Stefan Hajnoczi Sept. 29, 2020, 10:41 a.m. UTC | #5
On Tue, Sep 15, 2020 at 07:29:17AM -0700, Thanos Makatos wrote:
> This patch introduces the vfio-user protocol specification (formerly
> known as VFIO-over-socket), which is designed to allow devices to be
> emulated outside QEMU, in a separate process. vfio-user reuses the
> existing VFIO defines, structs and concepts.
> 
> It has been earlier discussed as an RFC in:
> "RFC: use VFIO over a UNIX domain socket to implement device offloading"
> 
> Signed-off-by: John G Johnson <john.g.johnson@oracle.com>
> Signed-off-by: Thanos Makatos <thanos.makatos@nutanix.com>

I reviewed the recently-added kernel VFIO_IOMMU_DIRTY_PAGES ioctl and
VFIO_REGION_TYPE_MIGRATION features. They enable live migration and
device state save/load.

Including them early would be good because it's difficult to retrofit
live migration into existing code later.

Stefan
John G Johnson Sept. 29, 2020, 4:21 p.m. UTC | #6
> On Sep 29, 2020, at 3:37 AM, Stefan Hajnoczi <stefanha@redhat.com> wrote:
> 
> On Mon, Sep 28, 2020 at 09:58:37AM +0000, Thanos Makatos wrote:
>>> It should be accompanied by a test in tests/. PCI-level testing APIS for
>>> BARs, configuration space, interrupts, etc are available in
>>> tests/qtest/libqos/pci.h. The test case needs to include a vfio-user
>>> device backend interact with QEMU's vfio-user-pci implementation.
>> 
>> We plan to use a libmuser-based backend for testing. This, I suppose, will
>> make libmuser a dependency of QEMU (either as a submodule or as a library),
>> which for now can be disabled in the default configuration. Is this acceptable?
> 
> If there are no other dependencies and libmuser supports all host
> operating systems that QEMU's -device vfio-user supports, then I think
> it's a good idea to use libmuser for at least one in-tree test in QEMU.
> 
>>> Also please let us know who is working on what so additional people can
>>> get involved in areas that need work!
>> 
>> Swapnil and I will be working on libmuser and the test in QEMU, John and
>> the mp-qemu folks will be working on the patches for implementing
>> --device vfio-user-pci.
> 
> Great!
> 
> John: Will mpqemu use libmuser to implement the remote PCI host
> controller?
> 


	The vfio-user-pci plan is to use libmuser on the server side.

								JJ
Stefan Hajnoczi Sept. 30, 2020, 2:24 p.m. UTC | #7
On Tue, Sep 29, 2020 at 09:21:54AM -0700, John G Johnson wrote:
> > On Sep 29, 2020, at 3:37 AM, Stefan Hajnoczi <stefanha@redhat.com> wrote:
> > 
> > On Mon, Sep 28, 2020 at 09:58:37AM +0000, Thanos Makatos wrote:
> >>> It should be accompanied by a test in tests/. PCI-level testing APIS for
> >>> BARs, configuration space, interrupts, etc are available in
> >>> tests/qtest/libqos/pci.h. The test case needs to include a vfio-user
> >>> device backend interact with QEMU's vfio-user-pci implementation.
> >> 
> >> We plan to use a libmuser-based backend for testing. This, I suppose, will
> >> make libmuser a dependency of QEMU (either as a submodule or as a library),
> >> which for now can be disabled in the default configuration. Is this acceptable?
> > 
> > If there are no other dependencies and libmuser supports all host
> > operating systems that QEMU's -device vfio-user supports, then I think
> > it's a good idea to use libmuser for at least one in-tree test in QEMU.
> > 
> >>> Also please let us know who is working on what so additional people can
> >>> get involved in areas that need work!
> >> 
> >> Swapnil and I will be working on libmuser and the test in QEMU, John and
> >> the mp-qemu folks will be working on the patches for implementing
> >> --device vfio-user-pci.
> > 
> > Great!
> > 
> > John: Will mpqemu use libmuser to implement the remote PCI host
> > controller?
> > 
> 
> 
> 	The vfio-user-pci plan is to use libmuser on the server side.

Okay. Using libmuser in tests seems like a good choice in that case.

We'll need to figure out the details of how to do it because the
traditional shared library dependency approach is not well-suited to
in-development code. It would involve shipping libmuser distro packages
so QEMU's build system can declare a library dependency (with details
provided in a pkg-config file).

Here are approaches that are better for in-development libraries:
1. Keep the libmuser code in qemu.git.
2. A copy of libmuser in qemu.git with changes being sent upstream
   (allows more flexibility in case QEMU-specific issues require
   experimentation).
3. Git submodules.

#1 if you're happy to use the QEMU development process for merging
libmuser code then it's easiest to officially host the code in qemu.git.
libmuser gets a subdirectory in the qemu.git tree and you (the
maintainers) send pull requests. A libmuser library build target
provides installable static and shared libraries so external
applications can link against libmuser too. The big advantage here is
that QEMU can instantly use the latest libmuser code changes.

#2 works best if the library is a small (just a few source files) with
no fancy build system requirements.

#3 is used in QEMU for several other components. Submodules are a pain
to sync (requires sending a qemu.git patch to move to a new commit ID),
so this isn't good for a dependency that moves quickly.

Stefan
Felipe Franciosi Oct. 2, 2020, 10:14 a.m. UTC | #8
> On Sep 30, 2020, at 3:24 PM, Stefan Hajnoczi <stefanha@redhat.com> wrote:
> 
> On Tue, Sep 29, 2020 at 09:21:54AM -0700, John G Johnson wrote:
>>> On Sep 29, 2020, at 3:37 AM, Stefan Hajnoczi <stefanha@redhat.com> wrote:
>>> 
>>> On Mon, Sep 28, 2020 at 09:58:37AM +0000, Thanos Makatos wrote:
>>>>> It should be accompanied by a test in tests/. PCI-level testing APIS for
>>>>> BARs, configuration space, interrupts, etc are available in
>>>>> tests/qtest/libqos/pci.h. The test case needs to include a vfio-user
>>>>> device backend interact with QEMU's vfio-user-pci implementation.
>>>> 
>>>> We plan to use a libmuser-based backend for testing. This, I suppose, will
>>>> make libmuser a dependency of QEMU (either as a submodule or as a library),
>>>> which for now can be disabled in the default configuration. Is this acceptable?
>>> 
>>> If there are no other dependencies and libmuser supports all host
>>> operating systems that QEMU's -device vfio-user supports, then I think
>>> it's a good idea to use libmuser for at least one in-tree test in QEMU.
>>> 
>>>>> Also please let us know who is working on what so additional people can
>>>>> get involved in areas that need work!
>>>> 
>>>> Swapnil and I will be working on libmuser and the test in QEMU, John and
>>>> the mp-qemu folks will be working on the patches for implementing
>>>> --device vfio-user-pci.
>>> 
>>> Great!
>>> 
>>> John: Will mpqemu use libmuser to implement the remote PCI host
>>> controller?
>>> 
>> 
>> 
>> 	The vfio-user-pci plan is to use libmuser on the server side.
> 
> Okay. Using libmuser in tests seems like a good choice in that case.
> 
> We'll need to figure out the details of how to do it because the
> traditional shared library dependency approach is not well-suited to
> in-development code. It would involve shipping libmuser distro packages
> so QEMU's build system can declare a library dependency (with details
> provided in a pkg-config file).
> 
> Here are approaches that are better for in-development libraries:
> 1. Keep the libmuser code in qemu.git.
> 2. A copy of libmuser in qemu.git with changes being sent upstream
>   (allows more flexibility in case QEMU-specific issues require
>   experimentation).
> 3. Git submodules.
> 
> #1 if you're happy to use the QEMU development process for merging
> libmuser code then it's easiest to officially host the code in qemu.git.
> libmuser gets a subdirectory in the qemu.git tree and you (the
> maintainers) send pull requests. A libmuser library build target
> provides installable static and shared libraries so external
> applications can link against libmuser too. The big advantage here is
> that QEMU can instantly use the latest libmuser code changes.

I think there's a couple of limitations here which we should keep in mind.

1. Does putting it in qemu.git precludes it being BSD-3?
There's been evidence of people using (or at least trying out) muser
from where it currently lives. That doesn't mean we can't move it, but
I'm wondering if it means we have to make it GPL.

2. What about other projects that need libmuser code?
What worries me more is projects like SPDK/DPDK wanting to link
against the library and having to clone the entire QEMU repo as a
submodule. That sounds a lot more expensive than option 3 and probably
have further complications if they aren't GPL.

> 
> #2 works best if the library is a small (just a few source files) with
> no fancy build system requirements.

The risk here is that they go out of sync. There's the same (or even
more) maintenance burden as point 3 below, with the added risk that
someone could patch the files and make cherry-picks non-trivial.

> 
> #3 is used in QEMU for several other components. Submodules are a pain
> to sync (requires sending a qemu.git patch to move to a new commit ID),
> so this isn't good for a dependency that moves quickly.

I argue this is no worse than option 2. It's what I think aligns best,
but let's keep weighing pros/cons and come to a conclusion together.
The list of maintainers for muser.git should be extended to include
more QEMU stakeholders and probably other projects that will use it
(as) heavily. The topic has been raised in SPDK's Slack team on
whether the client library should live in a repo of its own (eg.
libvfio-user.git). Given the reference implementation is in libmuser,
I still think muser.git is accurate (but can easily be persuaded
otherwise).

Cheers,
Felipe

> 
> Stefan
Stefan Hajnoczi Oct. 13, 2020, 9:30 a.m. UTC | #9
On Fri, Oct 02, 2020 at 10:14:23AM +0000, Felipe Franciosi wrote:
> 
> 
> > On Sep 30, 2020, at 3:24 PM, Stefan Hajnoczi <stefanha@redhat.com> wrote:
> > 
> > On Tue, Sep 29, 2020 at 09:21:54AM -0700, John G Johnson wrote:
> >>> On Sep 29, 2020, at 3:37 AM, Stefan Hajnoczi <stefanha@redhat.com> wrote:
> >>> 
> >>> On Mon, Sep 28, 2020 at 09:58:37AM +0000, Thanos Makatos wrote:
> >>>>> It should be accompanied by a test in tests/. PCI-level testing APIS for
> >>>>> BARs, configuration space, interrupts, etc are available in
> >>>>> tests/qtest/libqos/pci.h. The test case needs to include a vfio-user
> >>>>> device backend interact with QEMU's vfio-user-pci implementation.
> >>>> 
> >>>> We plan to use a libmuser-based backend for testing. This, I suppose, will
> >>>> make libmuser a dependency of QEMU (either as a submodule or as a library),
> >>>> which for now can be disabled in the default configuration. Is this acceptable?
> >>> 
> >>> If there are no other dependencies and libmuser supports all host
> >>> operating systems that QEMU's -device vfio-user supports, then I think
> >>> it's a good idea to use libmuser for at least one in-tree test in QEMU.
> >>> 
> >>>>> Also please let us know who is working on what so additional people can
> >>>>> get involved in areas that need work!
> >>>> 
> >>>> Swapnil and I will be working on libmuser and the test in QEMU, John and
> >>>> the mp-qemu folks will be working on the patches for implementing
> >>>> --device vfio-user-pci.
> >>> 
> >>> Great!
> >>> 
> >>> John: Will mpqemu use libmuser to implement the remote PCI host
> >>> controller?
> >>> 
> >> 
> >> 
> >> 	The vfio-user-pci plan is to use libmuser on the server side.
> > 
> > Okay. Using libmuser in tests seems like a good choice in that case.
> > 
> > We'll need to figure out the details of how to do it because the
> > traditional shared library dependency approach is not well-suited to
> > in-development code. It would involve shipping libmuser distro packages
> > so QEMU's build system can declare a library dependency (with details
> > provided in a pkg-config file).
> > 
> > Here are approaches that are better for in-development libraries:
> > 1. Keep the libmuser code in qemu.git.
> > 2. A copy of libmuser in qemu.git with changes being sent upstream
> >   (allows more flexibility in case QEMU-specific issues require
> >   experimentation).
> > 3. Git submodules.
> > 
> > #1 if you're happy to use the QEMU development process for merging
> > libmuser code then it's easiest to officially host the code in qemu.git.
> > libmuser gets a subdirectory in the qemu.git tree and you (the
> > maintainers) send pull requests. A libmuser library build target
> > provides installable static and shared libraries so external
> > applications can link against libmuser too. The big advantage here is
> > that QEMU can instantly use the latest libmuser code changes.
> 
> I think there's a couple of limitations here which we should keep in mind.
> 
> 1. Does putting it in qemu.git precludes it being BSD-3?
> There's been evidence of people using (or at least trying out) muser
> from where it currently lives. That doesn't mean we can't move it, but
> I'm wondering if it means we have to make it GPL.

The 3-clause BSD license is compatible with the GPL according to
Wikipedia:
https://en.wikipedia.org/wiki/BSD_licenses

> 2. What about other projects that need libmuser code?
> What worries me more is projects like SPDK/DPDK wanting to link
> against the library and having to clone the entire QEMU repo as a
> submodule. That sounds a lot more expensive than option 3 and probably
> have further complications if they aren't GPL.

In the early stages where the vfio-user protocol and library interfaces
might need changes it will be hard to use it from multiple applications
without compatibility issues. If SPDK/DPDK are communicating with QEMU
using a cutting-edge library then they probably need to build QEMU from
source anyway. ISTR they also maintain a QEMU fork? So maybe it's not a
big issue for them.

> > 
> > #2 works best if the library is a small (just a few source files) with
> > no fancy build system requirements.
> 
> The risk here is that they go out of sync. There's the same (or even
> more) maintenance burden as point 3 below, with the added risk that
> someone could patch the files and make cherry-picks non-trivial.
> 
> > 
> > #3 is used in QEMU for several other components. Submodules are a pain
> > to sync (requires sending a qemu.git patch to move to a new commit ID),
> > so this isn't good for a dependency that moves quickly.
> 
> I argue this is no worse than option 2. It's what I think aligns best,
> but let's keep weighing pros/cons and come to a conclusion together.
> The list of maintainers for muser.git should be extended to include
> more QEMU stakeholders and probably other projects that will use it
> (as) heavily. The topic has been raised in SPDK's Slack team on
> whether the client library should live in a repo of its own (eg.
> libvfio-user.git). Given the reference implementation is in libmuser,
> I still think muser.git is accurate (but can easily be persuaded
> otherwise).

Me too, no solution is perfect. My thoughts about developing it within
qemu.git for now is that this will make protocol and library interface
changes easy. It will also encourage applications (DPDK/SPDK) to build
against a matching QEMU so that there are no compatibility problems at
the protocol or library level while the code is still heavily under
development.

Stefan
Daniel P. Berrangé Oct. 13, 2020, 9:42 a.m. UTC | #10
On Wed, Sep 30, 2020 at 03:24:08PM +0100, Stefan Hajnoczi wrote:
> On Tue, Sep 29, 2020 at 09:21:54AM -0700, John G Johnson wrote:
> > > On Sep 29, 2020, at 3:37 AM, Stefan Hajnoczi <stefanha@redhat.com> wrote:
> > > 
> > > On Mon, Sep 28, 2020 at 09:58:37AM +0000, Thanos Makatos wrote:
> > >>> It should be accompanied by a test in tests/. PCI-level testing APIS for
> > >>> BARs, configuration space, interrupts, etc are available in
> > >>> tests/qtest/libqos/pci.h. The test case needs to include a vfio-user
> > >>> device backend interact with QEMU's vfio-user-pci implementation.
> > >> 
> > >> We plan to use a libmuser-based backend for testing. This, I suppose, will
> > >> make libmuser a dependency of QEMU (either as a submodule or as a library),
> > >> which for now can be disabled in the default configuration. Is this acceptable?
> > > 
> > > If there are no other dependencies and libmuser supports all host
> > > operating systems that QEMU's -device vfio-user supports, then I think
> > > it's a good idea to use libmuser for at least one in-tree test in QEMU.
> > > 
> > >>> Also please let us know who is working on what so additional people can
> > >>> get involved in areas that need work!
> > >> 
> > >> Swapnil and I will be working on libmuser and the test in QEMU, John and
> > >> the mp-qemu folks will be working on the patches for implementing
> > >> --device vfio-user-pci.
> > > 
> > > Great!
> > > 
> > > John: Will mpqemu use libmuser to implement the remote PCI host
> > > controller?
> > > 
> > 
> > 
> > 	The vfio-user-pci plan is to use libmuser on the server side.
> 
> Okay. Using libmuser in tests seems like a good choice in that case.
> 
> We'll need to figure out the details of how to do it because the
> traditional shared library dependency approach is not well-suited to
> in-development code. It would involve shipping libmuser distro packages
> so QEMU's build system can declare a library dependency (with details
> provided in a pkg-config file).
> 
> Here are approaches that are better for in-development libraries:
> 1. Keep the libmuser code in qemu.git.
> 2. A copy of libmuser in qemu.git with changes being sent upstream
>    (allows more flexibility in case QEMU-specific issues require
>    experimentation).
> 3. Git submodules.
> 
> #1 if you're happy to use the QEMU development process for merging
> libmuser code then it's easiest to officially host the code in qemu.git.
> libmuser gets a subdirectory in the qemu.git tree and you (the
> maintainers) send pull requests. A libmuser library build target
> provides installable static and shared libraries so external
> applications can link against libmuser too. The big advantage here is
> that QEMU can instantly use the latest libmuser code changes.
> 
> #2 works best if the library is a small (just a few source files) with
> no fancy build system requirements.
> 
> #3 is used in QEMU for several other components. Submodules are a pain
> to sync (requires sending a qemu.git patch to move to a new commit ID),
> so this isn't good for a dependency that moves quickly.

I don't think this is actually downside. If anything I think submodules
I think it would actually let libmuser develop faster.

QEMU is such a large project with high volume patches, that changes
take a long time to merge, even when everything is working well.

I think libmuser would have much faster development velocity if
the bit flowing into qemu.git was only as the trivial submodule
hash updates, as opposed to every single libmuser functional patch.

Maintaining libmuser as a standalone git repo avoids need for
libmuser to tie itself to QEMU's release cycle or processes. In
particular it means libmuser could opt to use a modern development
workflow using a GitForge with merge requests, instead of the
legacy email workflow.

Especially if we're considering libmuser to be consumed by non-QEMU
codebases, then long term it almost certainly should be a standalone
project. With this in mind, I think it will be better for libmuser
to start off as a standalone project straight away, so it can build
up its developer community organically, and not have people put off
contributing by the QEMU firehose.

Regards,
Daniel
Felipe Franciosi Oct. 15, 2020, 1:36 p.m. UTC | #11
Hi,

> On Oct 13, 2020, at 10:30 AM, Stefan Hajnoczi <stefanha@redhat.com> wrote:
> 
> On Fri, Oct 02, 2020 at 10:14:23AM +0000, Felipe Franciosi wrote:
>> 
>> 
>>> On Sep 30, 2020, at 3:24 PM, Stefan Hajnoczi <stefanha@redhat.com> wrote:
>>> 
>>> On Tue, Sep 29, 2020 at 09:21:54AM -0700, John G Johnson wrote:
>>>>> On Sep 29, 2020, at 3:37 AM, Stefan Hajnoczi <stefanha@redhat.com> wrote:
>>>>> 
>>>>> On Mon, Sep 28, 2020 at 09:58:37AM +0000, Thanos Makatos wrote:
>>>>>>> It should be accompanied by a test in tests/. PCI-level testing APIS for
>>>>>>> BARs, configuration space, interrupts, etc are available in
>>>>>>> tests/qtest/libqos/pci.h. The test case needs to include a vfio-user
>>>>>>> device backend interact with QEMU's vfio-user-pci implementation.
>>>>>> 
>>>>>> We plan to use a libmuser-based backend for testing. This, I suppose, will
>>>>>> make libmuser a dependency of QEMU (either as a submodule or as a library),
>>>>>> which for now can be disabled in the default configuration. Is this acceptable?
>>>>> 
>>>>> If there are no other dependencies and libmuser supports all host
>>>>> operating systems that QEMU's -device vfio-user supports, then I think
>>>>> it's a good idea to use libmuser for at least one in-tree test in QEMU.
>>>>> 
>>>>>>> Also please let us know who is working on what so additional people can
>>>>>>> get involved in areas that need work!
>>>>>> 
>>>>>> Swapnil and I will be working on libmuser and the test in QEMU, John and
>>>>>> the mp-qemu folks will be working on the patches for implementing
>>>>>> --device vfio-user-pci.
>>>>> 
>>>>> Great!
>>>>> 
>>>>> John: Will mpqemu use libmuser to implement the remote PCI host
>>>>> controller?
>>>>> 
>>>> 
>>>> 
>>>> 	The vfio-user-pci plan is to use libmuser on the server side.
>>> 
>>> Okay. Using libmuser in tests seems like a good choice in that case.
>>> 
>>> We'll need to figure out the details of how to do it because the
>>> traditional shared library dependency approach is not well-suited to
>>> in-development code. It would involve shipping libmuser distro packages
>>> so QEMU's build system can declare a library dependency (with details
>>> provided in a pkg-config file).
>>> 
>>> Here are approaches that are better for in-development libraries:
>>> 1. Keep the libmuser code in qemu.git.
>>> 2. A copy of libmuser in qemu.git with changes being sent upstream
>>>  (allows more flexibility in case QEMU-specific issues require
>>>  experimentation).
>>> 3. Git submodules.
>>> 
>>> #1 if you're happy to use the QEMU development process for merging
>>> libmuser code then it's easiest to officially host the code in qemu.git.
>>> libmuser gets a subdirectory in the qemu.git tree and you (the
>>> maintainers) send pull requests. A libmuser library build target
>>> provides installable static and shared libraries so external
>>> applications can link against libmuser too. The big advantage here is
>>> that QEMU can instantly use the latest libmuser code changes.
>> 
>> I think there's a couple of limitations here which we should keep in mind.
>> 
>> 1. Does putting it in qemu.git precludes it being BSD-3?
>> There's been evidence of people using (or at least trying out) muser
>> from where it currently lives. That doesn't mean we can't move it, but
>> I'm wondering if it means we have to make it GPL.
> 
> The 3-clause BSD license is compatible with the GPL according to
> Wikipedia:
> https://en.wikipedia.org/wiki/BSD_licenses

Ah, ok. That's not an issue then.

> 
>> 2. What about other projects that need libmuser code?
>> What worries me more is projects like SPDK/DPDK wanting to link
>> against the library and having to clone the entire QEMU repo as a
>> submodule. That sounds a lot more expensive than option 3 and probably
>> have further complications if they aren't GPL.
> 
> In the early stages where the vfio-user protocol and library interfaces
> might need changes it will be hard to use it from multiple applications
> without compatibility issues. If SPDK/DPDK are communicating with QEMU
> using a cutting-edge library then they probably need to build QEMU from
> source anyway. ISTR they also maintain a QEMU fork? So maybe it's not a
> big issue for them.

A few things on this:

Per Daniel's response on the other fork of the thread, I think we can
develop faster if we're not depending on qemu-devel. With the right
set of maintainers on the project, there's arguably more flexibility
in working at a restricted set of code. QEMU can then update the
submodule when a "checkpoint" is ready.

Also, I don't see why SPDK/DPDK would need to build QEMU from source
for their implementations. That is, they will need an up-to-date QEMU
for enabling their development, but they don't provide a QEMU to their
users; they just have to wait for a release.  I talked to Ben Walker
(SPDK), and he confirmed SPDK doesn't really maintain a fork of QEMU.
Apparently there was one when they were doing vhost-user-nvme, but
that's now abandoned.

If they had to add the entire QEMU as a submodule, it could
potentially be an issue, right? That's a lot of code just to get some
headers.

> 
>>> 
>>> #2 works best if the library is a small (just a few source files) with
>>> no fancy build system requirements.
>> 
>> The risk here is that they go out of sync. There's the same (or even
>> more) maintenance burden as point 3 below, with the added risk that
>> someone could patch the files and make cherry-picks non-trivial.
>> 
>>> 
>>> #3 is used in QEMU for several other components. Submodules are a pain
>>> to sync (requires sending a qemu.git patch to move to a new commit ID),
>>> so this isn't good for a dependency that moves quickly.
>> 
>> I argue this is no worse than option 2. It's what I think aligns best,
>> but let's keep weighing pros/cons and come to a conclusion together.
>> The list of maintainers for muser.git should be extended to include
>> more QEMU stakeholders and probably other projects that will use it
>> (as) heavily. The topic has been raised in SPDK's Slack team on
>> whether the client library should live in a repo of its own (eg.
>> libvfio-user.git). Given the reference implementation is in libmuser,
>> I still think muser.git is accurate (but can easily be persuaded
>> otherwise).
> 
> Me too, no solution is perfect. My thoughts about developing it within
> qemu.git for now is that this will make protocol and library interface
> changes easy. It will also encourage applications (DPDK/SPDK) to build
> against a matching QEMU so that there are no compatibility problems at
> the protocol or library level while the code is still heavily under
> development.

I suppose that challenge is up to the vfio-user/libmuser maintainers
to resolve by tagging rc versions or something along those lines.
Those would be the ones we can add to QEMU master as the work
progresses.

Thoughts?
F.

> 
> Stefan
Stefan Hajnoczi Oct. 30, 2020, 7:14 p.m. UTC | #12
On Thu, Oct 15, 2020 at 2:39 PM Felipe Franciosi <felipe@nutanix.com> wrote:
> > On Oct 13, 2020, at 10:30 AM, Stefan Hajnoczi <stefanha@redhat.com> wrote:> >
> > On Fri, Oct 02, 2020 at 10:14:23AM +0000, Felipe Franciosi wrote:
> >>> On Sep 30, 2020, at 3:24 PM, Stefan Hajnoczi <stefanha@redhat.com> wrote:
> >>> On Tue, Sep 29, 2020 at 09:21:54AM -0700, John G Johnson wrote:
> >>>>> On Sep 29, 2020, at 3:37 AM, Stefan Hajnoczi <stefanha@redhat.com> wrote:
> >>>>>
> >>>>> On Mon, Sep 28, 2020 at 09:58:37AM +0000, Thanos Makatos wrote:
> Per Daniel's response on the other fork of the thread, I think we can
> develop faster if we're not depending on qemu-devel. With the right
> set of maintainers on the project, there's arguably more flexibility
> in working at a restricted set of code. QEMU can then update the
> submodule when a "checkpoint" is ready.

Sure, if you are happy with the submodule approach that's great.

On the QEMU side the device name should have an "x-" prefix so it's
clear that the feature is experimental and subject to change. This way
there is no stability guarantee during development. Both the protocol
and command-line can be changed without introducing feature bits and
keeping backwards compatibility.

Stefan
diff mbox series

Patch

diff --git a/MAINTAINERS b/MAINTAINERS
index 030faf0..a7f4b8f 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -1732,6 +1732,12 @@  F: hw/vfio/ap.c
 F: docs/system/s390x/vfio-ap.rst
 L: qemu-s390x@nongnu.org
 
+vfio-user
+M: John G Johnson <john.g.johnson@oracle.com>
+M: Thanos Makatos <thanos.makatos@nutanix.com>
+S: Supported
+F: docs/devel/vfio-user.rst
+
 vhost
 M: Michael S. Tsirkin <mst@redhat.com>
 S: Supported
diff --git a/docs/devel/index.rst b/docs/devel/index.rst
index ae6eac7..e6a89c2 100644
--- a/docs/devel/index.rst
+++ b/docs/devel/index.rst
@@ -30,3 +30,4 @@  Contents:
    reset
    s390-dasd-ipl
    clocks
+   vfio-user
diff --git a/docs/devel/vfio-user.rst b/docs/devel/vfio-user.rst
new file mode 100644
index 0000000..0217270
--- /dev/null
+++ b/docs/devel/vfio-user.rst
@@ -0,0 +1,1191 @@ 
+********************************
+vfio-user Protocol Specification
+********************************
+
+------------
+Version_ 0.1
+------------
+
+.. contents:: Table of Contents
+
+Introduction
+============
+vfio-user is a protocol that allows a device to be emulated in a separate
+process outside of a Virtual Machine Monitor (VMM). vfio-user devices consist
+of a generic VFIO device type, living inside the VMM, which we call the client,
+and the core device implementation, living outside the VMM, which we call the
+server.
+
+The `Linux VFIO ioctl interface <https://www.kernel.org/doc/html/latest/driver-api/vfio.html>`_
+been chosen as the base for this protocol for the following reasons:
+
+1) It is a mature and stable API, backed by an extensively used framework.
+2) The existing VFIO client implementation in QEMU (qemu/hw/vfio/) can be
+   largely reused.
+
+.. Note::
+   In a proof of concept implementation it has been demonstrated that using VFIO
+   over a UNIX domain socket is a viable option. vfio-user is designed with
+   QEMU in mind, however it could be used by other client applications. The
+   vfio-user protocol does not require that QEMU's VFIO client  implementation
+   is used in QEMU.
+
+None of the VFIO kernel modules are required for supporting the protocol,
+neither in the client nor the server, only the source header files are used.
+
+The main idea is to allow a virtual device to function in a separate process in
+the same host over a UNIX domain socket. A UNIX domain socket (AF_UNIX) is
+chosen because file descriptors can be trivially sent over it, which in turn
+allows:
+
+* Sharing of client memory for DMA with the server.
+* Sharing of server memory with the client for fast MMIO.
+* Efficient sharing of eventfd's for triggering interrupts.
+
+Other socket types could be used which allow the server to run in a separate
+guest in the same host (AF_VSOCK) or remotely (AF_INET). Theoretically the
+underlying transport does not necessarily have to be a socket, however we do
+not examine such alternatives. In this protocol version we focus on using a
+UNIX domain socket and introduce basic support for the other two types of
+sockets without considering performance implications.
+
+While passing of file descriptors is desirable for performance reasons, it is
+not necessary neither for the client nor for the server to support it in order
+to implement the protocol. There is always an in-band, message-passing fall
+back mechanism.
+
+VFIO
+====
+VFIO is a framework that allows a physical device to be securely passed through
+to a user space process; the device-specific kernel driver does not drive the
+device at all.  Typically, the user space process is a VMM and the device is
+passed through to it in order to achieve high performance. VFIO provides an API
+and the required functionality in the kernel. QEMU has adopted VFIO to allow a
+guest to directly access physical devices, instead of emulating them in
+software.
+
+vfio-user reuses the core VFIO concepts defined in its API, but implements them
+as messages to be sent over a socket. It does not change the kernel-based VFIO
+in any way, in fact none of the VFIO kernel modules need to be loaded to use
+vfio-user. It is also possible for the client to concurrently use the current
+kernel-based VFIO for one device, and vfio-user for another device.
+
+VFIO Device Model
+-----------------
+A device under VFIO presents a standard interface to the user process. Many of
+the VFIO operations in the existing interface use the ioctl() system call, and
+references to the existing interface are called the ioctl() implementation in
+this document.
+
+The following sections describe the set of messages that implement the VFIO
+interface over a socket. In many cases, the messages are direct translations of
+data structures used in the ioctl() implementation. Messages derived from
+ioctl()s will have a name derived from the ioctl() command name.  E.g., the
+VFIO_GET_INFO ioctl() command becomes a VFIO_USER_GET_INFO message.  The
+purpose of this reuse is to share as much code as feasible with the ioctl()
+implementation.
+
+Connection Initiation
+^^^^^^^^^^^^^^^^^^^^^
+After the client connects to the server, the initial server message is
+VFIO_USER_VERSION to propose a protocol version and set of capabilities to
+apply to the session. The client replies with a compatible version and set of
+capabilities it supports, or closes the connection if it cannot support the
+advertised version.
+
+DMA Memory Configuration
+^^^^^^^^^^^^^^^^^^^^^^^^
+The client uses VFIO_USER_DMA_MAP and VFIO_USER_DMA_UNMAP messages to inform
+the server of the valid DMA ranges that the server can access on behalf
+of a device. DMA memory may be accessed by the server via VFIO_USER_DMA_READ
+and VFIO_USER_DMA_WRITE messages over the socket.
+
+An optimization for server access to client memory is for the client to provide
+file descriptors the server can mmap() to directly access client memory. Note
+that mmap() privileges cannot be revoked by the client, therefore file
+descriptors should only be exported in environments where the client trusts the
+server not to corrupt guest memory.
+
+Device Information
+^^^^^^^^^^^^^^^^^^
+The client uses a VFIO_USER_DEVICE_GET_INFO message to query the server for
+information about the device. This information includes:
+
+* The device type and capabilities,
+* the number of device regions, and
+* the device presents to the client the number of interrupt types the device
+  supports.
+
+Region Information
+^^^^^^^^^^^^^^^^^^
+The client uses VFIO_USER_DEVICE_GET_REGION_INFO messages to query the server
+for information about the device's memory regions. This information describes:
+
+* Read and write permissions, whether it can be memory mapped, and whether it
+  supports additional capabilities.
+* Region index, size, and offset.
+
+When a region can be mapped by the client, the server provides a file
+descriptor which the client can mmap(). The server is responsible for polling
+for client updates to memory mapped regions.
+
+Region Capabilities
+"""""""""""""""""""
+Some regions have additional capabilities that cannot be described adequately
+by the region info data structure. These capabilities are returned in the
+region info reply in a list similar to PCI capabilities in a PCI device's
+configuration space.
+
+Sparse Regions
+""""""""""""""
+A region can be memory-mappable in whole or in part. When only a subset of a
+region can be mapped by the client, a VFIO_REGION_INFO_CAP_SPARSE_MMAP
+capability is included in the region info reply. This capability describes
+which portions can be mapped by the client.
+
+.. Note::
+   For example, in a virtual NVMe controller, sparse regions can be used so
+   that accesses to the NVMe registers (found in the beginning of BAR0) are
+   trapped (an infrequent event), while allowing direct access to the doorbells
+   (an extremely frequent event as every I/O submission requires a write to
+   BAR0), found right after the NVMe registers in BAR0.
+
+Interrupts
+^^^^^^^^^^
+The client uses VFIO_USER_DEVICE_GET_IRQ_INFO messages to query the server for
+the device's interrupt types. The interrupt types are specific to the bus the
+device is attached to, and the client is expected to know the capabilities of
+each interrupt type. The server can signal an interrupt either with
+VFIO_USER_VM_INTERRUPT messages over the socket, or can directly inject
+interrupts into the guest via an event file descriptor. The client configures
+how the server signals an interrupt with VFIO_USER_SET_IRQS messages.
+
+Device Read and Write
+^^^^^^^^^^^^^^^^^^^^^
+When the guest executes load or store operations to device memory, the client
+forwards these operations to the server with VFIO_USER_REGION_READ or
+VFIO_USER_REGION_WRITE messages. The server will reply with data from the
+device on read operations or an acknowledgement on write operations.
+
+DMA
+^^^
+When a device performs DMA accesses to guest memory, the server will forward
+them to the client with VFIO_USER_DMA_READ and VFIO_USER_DMA_WRITE messages.
+These messages can only be used to access guest memory the client has
+configured into the server.
+
+Protocol Specification
+======================
+To distinguish from the base VFIO symbols, all vfio-user symbols are prefixed
+with vfio_user or VFIO_USER. In revision 0.1, all data is in the little-endian
+format, although this may be relaxed in future revision in cases where the
+client and server are both big-endian. The messages are formatted for seamless
+reuse of the native VFIO structs.
+
+Socket
+------
+
+A server can serve:
+
+1) one or more clients, and/or
+2) one or more virtual devices, belonging to one or more clients.
+
+The current protocol specification requires a dedicated socket per
+client/server connection. It is a server-side implementation detail whether a
+single server handles multiple virtual devices from the same or multiple
+clients. The location of the socket is implementation-specific. Multiplexing
+clients, devices, and servers over the same socket is not supported in this
+version of the protocol.
+
+Authentication
+--------------
+For AF_UNIX, we rely on OS mandatory access controls on the socket files,
+therefore it is up to the management layer to set up the socket as required.
+Socket types than span guests or hosts will require a proper authentication
+mechanism. Defining that mechanism is deferred to a future version of the
+protocol.
+
+Command Concurrency
+-------------------
+A client may pipeline multiple commands without waiting for previous command
+replies.  The server will process commands in the order they are received.
+A consequence of this is if a client issues a command with the *No_reply* bit,
+then subseqently issues a command without *No_reply*, the older command will
+have been processed before the reply to the younger command is sent by the
+server.  The client must be aware of the device's capability to process concurrent
+commands if pipelining is used.  For example, pipelining allows multiple client
+threads to concurently access device memory; the client must ensure these acceses
+obey device semantics.
+
+An example is a frame buffer device, where the device may allow concurrent access
+to different areas of video memory, but may have indeterminate behavior if concurrent
+acceses are performed to command or status registers.
+
+Socket Disconnection Behavior
+-----------------------------
+The server and the client can disconnect from each other, either intentionally
+or unexpectedly. Both the client and the server need to know how to handle such
+events.
+
+Server Disconnection
+^^^^^^^^^^^^^^^^^^^^
+A server disconnecting from the client may indicate that:
+
+1) A virtual device has been restarted, either intentionally (e.g. because of a
+   device update) or unintentionally (e.g. because of a crash).
+2) A virtual device has been shut down with no intention to be restarted.
+
+It is impossible for the client to know whether or not a failure is
+intermittent or innocuous and should be retried, therefore the client should
+reset the VFIO device when it detects the socket has been disconnected.
+Error recovery will be driven by the guest's device error handling
+behavior.
+
+Client Disconnection
+^^^^^^^^^^^^^^^^^^^^
+The client disconnecting from the server primarily means that the client
+has exited. Currently, this means that the guest is shut down so the device is
+no longer needed therefore the server can automatically exit. However, there
+can be cases where a client disconnection should not result in a server exit:
+
+1) A single server serving multiple clients.
+2) A multi-process QEMU upgrading itself step by step, which is not yet
+   implemented.
+
+Therefore in order for the protocol to be forward compatible the server should
+take no action when the client disconnects. If anything happens to the client
+the control stack will know about it and can clean up resources
+accordingly.
+
+Live Migration
+^^^^^^^^^^^^^^
+A future version of the protocol will support client live migration.  This action
+will require the socket to be quiesced before it is disconnected,  This mechanism
+will be defined when live migration support is added.
+
+Request Retry and Response Timeout
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+A failed command is a command that has been successfully sent and has been
+responded to with an error code. Failure to send the command in the first place
+(e.g. because the socket is disconnected) is a different type of error examined
+earlier in the disconnect section.
+
+.. Note::
+   QEMU's VFIO retries certain operations if they fail. While this makes sense
+   for real HW, we don't know for sure whether it makes sense for virtual
+   devices.
+
+Defining a retry and timeout scheme is deferred to a future version of the
+protocol.
+
+.. _Commands:
+
+Commands
+--------
+The following table lists the VFIO message command IDs, and whether the
+message command is sent from the client or the server.
+
++----------------------------------+---------+-------------------+
+| Name                             | Command | Request Direction |
++==================================+=========+===================+
+| VFIO_USER_VERSION                | 1       | server -> client  |
++----------------------------------+---------+-------------------+
+| VFIO_USER_DMA_MAP                | 2       | client -> server  |
++----------------------------------+---------+-------------------+
+| VFIO_USER_DMA_UNMAP              | 3       | client -> server  |
++----------------------------------+---------+-------------------+
+| VFIO_USER_DEVICE_GET_INFO        | 4       | client -> server  |
++----------------------------------+---------+-------------------+
+| VFIO_USER_DEVICE_GET_REGION_INFO | 5       | client -> server  |
++----------------------------------+---------+-------------------+
+| VFIO_USER_DEVICE_GET_IRQ_INFO    | 6       | client -> server  |
++----------------------------------+---------+-------------------+
+| VFIO_USER_DEVICE_SET_IRQS        | 7       | client -> server  |
++----------------------------------+---------+-------------------+
+| VFIO_USER_REGION_READ            | 8       | client -> server  |
++----------------------------------+---------+-------------------+
+| VFIO_USER_REGION_WRITE           | 9       | client -> server  |
++----------------------------------+---------+-------------------+
+| VFIO_USER_DMA_READ               | 10      | server -> client  |
++----------------------------------+---------+-------------------+
+| VFIO_USER_DMA_WRITE              | 11      | server -> client  |
++----------------------------------+---------+-------------------+
+| VFIO_USER_VM_INTERRUPT           | 12      | server -> client  |
++----------------------------------+---------+-------------------+
+| VFIO_USER_DEVICE_RESET           | 13      | client -> server  |
++----------------------------------+---------+-------------------+
+
+.. Note:: Some VFIO defines cannot be reused since their values are
+   architecture-specific (e.g. VFIO_IOMMU_MAP_DMA).
+
+Header
+------
+All messages, both command messages and reply messages, are preceded by a
+header that contains basic information about the message. The header is
+followed by message-specific data described in the sections below.
+
++----------------+--------+-------------+
+| Name           | Offset | Size        |
++================+========+=============+
+| Message ID     | 0      | 2           |
++----------------+--------+-------------+
+| Command        | 2      | 2           |
++----------------+--------+-------------+
+| Message size   | 4      | 4           |
++----------------+--------+-------------+
+| Flags          | 8      | 4           |
++----------------+--------+-------------+
+|                | +-----+------------+ |
+|                | | Bit | Definition | |
+|                | +=====+============+ |
+|                | | 0-3 | Type       | |
+|                | +-----+------------+ |
+|                | | 4   | No_reply   | |
+|                | +-----+------------+ |
+|                | | 5   | Error      | |
+|                | +-----+------------+ |
++----------------+--------+-------------+
+| Error          | 12     | 4           |
++----------------+--------+-------------+
+| <message data> | 16     | variable    |
++----------------+--------+-------------+
+
+* *Message ID* identifies the message, and is echoed in the command's reply message.
+* *Command* specifies the command to be executed, listed in Commands_.
+* *Message size* contains the size of the entire message, including the header.
+* *Flags* contains attributes of the message:
+
+  * The *Type* bits indicate the message type.
+
+    *  *Command* (value 0x0) indicates a command message.
+    *  *Reply* (value 0x1) indicates a reply message acknowledging a previous
+       command with the same message ID.
+  * *No_reply* in a command message indicates that no reply is needed for this command.
+    This is commonly used when multiple commands are sent, and only the last needs
+    acknowledgement.
+  * *Error* in a reply message indicates the command being acknowledged had
+    an error. In this case, the *Error* field will be valid.
+
+* *Error* in a reply message is a UNIX errno value. It is reserved in a command message.
+
+Each command message in Commands_ must be replied to with a reply message, unless the
+message sets the *No_Reply* bit.  The reply consists of the header with the *Reply*
+bit set, plus any additional data.
+
+VFIO_USER_VERSION
+-----------------
+
+Message format
+^^^^^^^^^^^^^^
+
++--------------+------------------------+
+| Name         | Value                  |
++==============+========================+
+| Message ID   | <ID>                   |
++--------------+------------------------+
+| Command      | 1                      |
++--------------+------------------------+
+| Message size | 16 + version length    |
++--------------+------------------------+
+| Flags        | Reply bit set in reply |
++--------------+------------------------+
+| Error        | 0/errno                |
++--------------+------------------------+
+| Version      | JSON byte array        |
++--------------+------------------------+
+
+This is the initial message sent by the server after the socket connection is
+established. The version is in JSON format, and the following objects must be
+included:
+
++--------------+--------+---------------------------------------------------+
+| Name         | Type   | Description                                       |
++==============+========+===================================================+
+| version      | object | ``{"major": <number>, "minor": <number>}``        |
+|              |        |                                                   |
+|              |        | Version supported by the sender, e.g. "0.1".      |
++--------------+--------+---------------------------------------------------+
+| capabilities | array  | Reserved. Can be omitted for v0.1, otherwise must |
+|              |        | be empty.                                         |
++--------------+--------+---------------------------------------------------+
+
+.. _Version:
+
+Versioning and Feature Support
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+Upon accepting a connection, the server must send a VFIO_USER_VERSION message
+proposing a protocol version and a set of capabilities. The client compares
+these with the versions and capabilities it supports and sends a
+VFIO_USER_VERSION reply according to the following rules.
+
+* The major version in the reply must be the same as proposed. If the client
+  does not support the proposed major, it closes the connection.
+* The minor version in the reply must be equal to or less than the minor
+  version proposed.
+* The capability list must be a subset of those proposed. If the client
+  requires a capability the server did not include, it closes the connection.
+
+The protocol major version will only change when incompatible protocol changes
+are made, such as changing the message format. The minor version may change
+when compatible changes are made, such as adding new messages or capabilities,
+Both the client and server must support all minor versions less than the
+maximum minor version it supports. E.g., an implementation that supports
+version 1.3 must also support 1.0 through 1.2.
+
+When making a change to this specification, the protocol version number must
+be included in the form "added in version X.Y"
+
+
+VFIO_USER_DMA_MAP and VFIO_USER_DMA_UNMAP
+-----------------------------------------
+
+Message Format
+^^^^^^^^^^^^^^
+
++--------------+------------------------+
+| Name         | Value                  |
++==============+========================+
+| Message ID   | <ID>                   |
++--------------+------------------------+
+| Command      | MAP=2, UNMAP=3         |
++--------------+------------------------+
+| Message size | 16 + table size        |
++--------------+------------------------+
+| Flags        | Reply bit set in reply |
++--------------+------------------------+
+| Error        | 0/errno                |
++--------------+------------------------+
+| Table        | array of table entries |
++--------------+------------------------+
+
+This command message is sent by the client to the server to inform it of the
+memory regions the server can access. It must be sent before the server can
+perform any DMA to the client. It is normally sent directly after the version
+handshake is completed, but may also occur when memory is added to or
+subtracted from the client, or if the client uses a vIOMMU. If the client does
+not expect the server to perform DMA then it does not need to send to the
+server VFIO_USER_DMA_MAP and VFIO_USER_DMA_UNMAP commands. If the server does
+not need to perform DMA the then it can ignore such commands but it must still
+reply to them. The table is an array of the following structure.  This
+structure is 32 bytes in size, so the message size is:
+16 + (# of table entries * 32).
+
+Table entry format
+^^^^^^^^^^^^^^^^^^
+
++-------------+--------+-------------+
+| Name        | Offset | Size        |
++=============+========+=============+
+| Address     | 0      | 8           |
++-------------+--------+-------------+
+| Size        | 8      | 8           |
++-------------+--------+-------------+
+| Offset      | 16     | 8           |
++-------------+--------+-------------+
+| Protections | 24     | 4           |
++-------------+--------+-------------+
+| Flags       | 28     | 4           |
++-------------+--------+-------------+
+|             | +-----+------------+ |
+|             | | Bit | Definition | |
+|             | +=====+============+ |
+|             | | 0   | Mappable   | |
+|             | +-----+------------+ |
++-------------+--------+-------------+
+
+* *Address* is the base DMA address of the region.
+* *Size* is the size of the region.
+* *Offset* is the file offset of the region with respect to the associated file
+  descriptor.
+* *Protections* are the region's protection attributes as encoded in
+  ``<sys/mman.h>``.
+* *Flags* contain the following region attributes:
+
+  * *Mappable* indicates that the region can be mapped via the mmap() system call
+    using the file descriptor provided in the message meta-data.
+
+VFIO_USER_DMA_MAP
+"""""""""""""""""
+If a DMA region being added can be directly mapped by the server, an array of
+file descriptors must be sent as part of the message meta-data. Each region
+entry must have a corresponding file descriptor. On AF_UNIX sockets, the file
+descriptors must be passed as SCM_RIGHTS type ancillary data. Otherwise, if a
+DMA region cannot be directly mapped by the server, it can be accessed by the
+server using VFIO_USER_DMA_READ and VFIO_USER_DMA_WRITE messages, explained in
+`Read and Write Operations`_. A command to map over an existing region must be
+failed by the server with ``EEXIST`` set in error field in the reply.
+
+VFIO_USER_DMA_UNMAP
+"""""""""""""""""""
+Upon receiving a VFIO_USER_DMA_UNMAP command, if the file descriptor is mapped
+then the server must release all references to that DMA region before replying,
+which includes potentially in flight DMA transactions. Removing a portion of a
+DMA region is possible.
+
+VFIO_USER_DEVICE_GET_INFO
+-------------------------
+
+Message format
+^^^^^^^^^^^^^^
+
++--------------+----------------------------+
+| Name         | Value                      |
++==============+============================+
+| Message ID   | <ID>                       |
++--------------+----------------------------+
+| Command      | 4                          |
++--------------+----------------------------+
+| Message size | 16 in command, 32 in reply |
++--------------+----------------------------+
+| Flags        | Reply bit set in reply     |
++--------------+----------------------------+
+| Error        | 0/errno                    |
++--------------+----------------------------+
+| Device info  | VFIO device info           |
++--------------+----------------------------+
+
+This command message is sent by the client to the server to query for basic
+information about the device. Only the message header is needed in the command
+message.  The VFIO device info structure is defined in ``<linux/vfio.h>``
+(``struct vfio_device_info``).
+
+VFIO device info format
+^^^^^^^^^^^^^^^^^^^^^^^
+
++-------------+--------+--------------------------+
+| Name        | Offset | Size                     |
++=============+========+==========================+
+| argsz       | 16     | 4                        |
++-------------+--------+--------------------------+
+| flags       | 20     | 4                        |
++-------------+--------+--------------------------+
+|             | +-----+-------------------------+ |
+|             | | Bit | Definition              | |
+|             | +=====+=========================+ |
+|             | | 0   | VFIO_DEVICE_FLAGS_RESET | |
+|             | +-----+-------------------------+ |
+|             | | 1   | VFIO_DEVICE_FLAGS_PCI   | |
+|             | +-----+-------------------------+ |
++-------------+--------+--------------------------+
+| num_regions | 24     | 4                        |
++-------------+--------+--------------------------+
+| num_irqs    | 28     | 4                        |
++-------------+--------+--------------------------+
+
+* *argsz* is the size of the VFIO device info structure.
+* *flags* contains the following device attributes.
+
+  * VFIO_DEVICE_FLAGS_RESET indicates that the device supports the
+    VFIO_USER_DEVICE_RESET message.
+  * VFIO_DEVICE_FLAGS_PCI indicates that the device is a PCI device.
+
+* *num_regions* is the number of memory regions that the device exposes.
+* *num_irqs* is the number of distinct interrupt types that the device supports.
+
+This version of the protocol only supports PCI devices. Additional devices may
+be supported in future versions.
+
+VFIO_USER_DEVICE_GET_REGION_INFO
+--------------------------------
+
+Message format
+^^^^^^^^^^^^^^
+
++--------------+------------------------+
+| Name         | Value                  |
++==============+========================+
+| Message ID   | <ID>                   |
++--------------+------------------------+
+| Command      | 5                      |
++--------------+------------------------+
+| Message size | 48 + any caps          |
++--------------+------------------------+
+| Flags        | Reply bit set in reply |
++--------------+------------------------+
+| Error        | 0/errno                |
++--------------+------------------------+
+| Region info  | VFIO region info       |
++--------------+------------------------+
+
+This command message is sent by the client to the server to query for
+information about device memory regions. The VFIO region info structure is
+defined in ``<linux/vfio.h>`` (``struct vfio_region_info``). Since the client
+does not know the size of the capabilities, the size of the reply it should
+expect is 48 plus any capabilities whose size is indicated in the size field of
+the reply header.
+
+VFIO region info format
+^^^^^^^^^^^^^^^^^^^^^^^
+
++------------+--------+------------------------------+
+| Name       | Offset | Size                         |
++============+========+==============================+
+| argsz      | 16     | 4                            |
++------------+--------+------------------------------+
+| flags      | 20     | 4                            |
++------------+--------+------------------------------+
+|            | +-----+-----------------------------+ |
+|            | | Bit | Definition                  | |
+|            | +=====+=============================+ |
+|            | | 0   | VFIO_REGION_INFO_FLAG_READ  | |
+|            | +-----+-----------------------------+ |
+|            | | 1   | VFIO_REGION_INFO_FLAG_WRITE | |
+|            | +-----+-----------------------------+ |
+|            | | 2   | VFIO_REGION_INFO_FLAG_MMAP  | |
+|            | +-----+-----------------------------+ |
+|            | | 3   | VFIO_REGION_INFO_FLAG_CAPS  | |
+|            | +-----+-----------------------------+ |
++------------+--------+------------------------------+
+| index      | 24     | 4                            |
++------------+--------+------------------------------+
+| cap_offset | 28     | 4                            |
++------------+--------+------------------------------+
+| size       | 32     | 8                            |
++------------+--------+------------------------------+
+| offset     | 40     | 8                            |
++------------+--------+------------------------------+
+
+* *argsz* is the size of the VFIO region info structure plus the
+  size of any region capabilities returned.
+* *flags* are attributes of the region:
+
+  * *VFIO_REGION_INFO_FLAG_READ* allows client read access to the region.
+  * *VFIO_REGION_INFO_FLAG_WRITE* allows client write access to the region.
+  * *VFIO_REGION_INFO_FLAG_MMAP* specifies the client can mmap() the region.
+    When this flag is set, the reply will include a file descriptor in its
+    meta-data. On AF_UNIX sockets, the file descriptors will be passed as
+    SCM_RIGHTS type ancillary data.
+  * *VFIO_REGION_INFO_FLAG_CAPS* indicates additional capabilities found in the
+    reply.
+
+* *index* is the index of memory region being queried, it is the only field
+  that is required to be set in the command message.
+* *cap_offset* describes where additional region capabilities can be found.
+  cap_offset is relative to the beginning of the VFIO region info structure.
+  The data structure it points is a VFIO cap header defined in
+  ``<linux/vfio.h>``.
+* *size* is the size of the region.
+* *offset* is the offset given to the mmap() system call for regions with the
+  MMAP attribute. It is also used as the base offset when mapping a VFIO
+  sparse mmap area, described below.
+
+VFIO Region capabilities
+^^^^^^^^^^^^^^^^^^^^^^^^
+The VFIO region information can also include a capabilities list. This list is
+similar to a PCI capability list - each entry has a common header that
+identifies a capability and where the next capability in the list can be found.
+The VFIO capability header format is defined in ``<linux/vfio.h>`` (``struct
+vfio_info_cap_header``).
+
+VFIO cap header format
+^^^^^^^^^^^^^^^^^^^^^^
+
++---------+--------+------+
+| Name    | Offset | Size |
++=========+========+======+
+| id      | 0      | 2    |
++---------+--------+------+
+| version | 2      | 2    |
++---------+--------+------+
+| next    | 4      | 4    |
++---------+--------+------+
+
+* *id* is the capability identity.
+* *version* is a capability-specific version number.
+* *next* specifies the offset of the next capability in the capability list. It
+  is relative to the beginning of the VFIO region info structure.
+
+VFIO sparse mmap
+^^^^^^^^^^^^^^^^
+
++------------------+----------------------------------+
+| Name             | Value                            |
++==================+==================================+
+| id               | VFIO_REGION_INFO_CAP_SPARSE_MMAP |
++------------------+----------------------------------+
+| version          | 0x1                              |
++------------------+----------------------------------+
+| next             | <next>                           |
++------------------+----------------------------------+
+| sparse mmap info | VFIO region info sparse mmap     |
++------------------+----------------------------------+
+
+The only capability supported in this version of the protocol is for sparse
+mmap. This capability is defined when only a subrange of the region supports
+direct access by the client via mmap(). The VFIO sparse mmap area is defined in
+``<linux/vfio.h>`` (``struct vfio_region_sparse_mmap_area``).
+
+VFIO region info cap sparse mmap
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
++----------+--------+------+
+| Name     | Offset | Size |
++==========+========+======+
+| nr_areas | 0      | 4    |
++----------+--------+------+
+| reserved | 4      | 4    |
++----------+--------+------+
+| offset   | 8      | 8    |
++----------+--------+------+
+| size     | 16     | 9    |
++----------+--------+------+
+| ...      |        |      |
++----------+--------+------+
+
+* *nr_areas* is the number of sparse mmap areas in the region.
+* *offset* and size describe a single area that can be mapped by the client.
+  There will be nr_areas pairs of offset and size. The offset will be added to
+  the base offset given in the VFIO_USER_DEVICE_GET_REGION_INFO to form the
+  offset argument of the subsequent mmap() call.
+
+The VFIO sparse mmap area is defined in ``<linux/vfio.h>`` (``struct
+vfio_region_info_cap_sparse_mmap``).
+
+VFIO_USER_DEVICE_GET_IRQ_INFO
+-----------------------------
+
+Message format
+^^^^^^^^^^^^^^
+
++--------------+------------------------+
+| Name         | Value                  |
++==============+========================+
+| Message ID   | <ID>                   |
++--------------+------------------------+
+| Command      | 6                      |
++--------------+------------------------+
+| Message size | 32                     |
++--------------+------------------------+
+| Flags        | Reply bit set in reply |
++--------------+------------------------+
+| Error        | 0/errno                |
++--------------+------------------------+
+| IRQ info     | VFIO IRQ info          |
++--------------+------------------------+
+
+This command message is sent by the client to the server to query for
+information about device interrupt types. The VFIO IRQ info structure is
+defined in ``<linux/vfio.h>`` (``struct vfio_irq_info``).
+
+VFIO IRQ info format
+^^^^^^^^^^^^^^^^^^^^
+
++-------+--------+---------------------------+
+| Name  | Offset | Size                      |
++=======+========+===========================+
+| argsz | 16     | 4                         |
++-------+--------+---------------------------+
+| flags | 20     | 4                         |
++-------+--------+---------------------------+
+|       | +-----+--------------------------+ |
+|       | | Bit | Definition               | |
+|       | +=====+==========================+ |
+|       | | 0   | VFIO_IRQ_INFO_EVENTFD    | |
+|       | +-----+--------------------------+ |
+|       | | 1   | VFIO_IRQ_INFO_MASKABLE   | |
+|       | +-----+--------------------------+ |
+|       | | 2   | VFIO_IRQ_INFO_AUTOMASKED | |
+|       | +-----+--------------------------+ |
+|       | | 3   | VFIO_IRQ_INFO_NORESIZE   | |
+|       | +-----+--------------------------+ |
++-------+--------+---------------------------+
+| index | 24     | 4                         |
++-------+--------+---------------------------+
+| count | 28     | 4                         |
++-------+--------+---------------------------+
+
+* *argsz* is the size of the VFIO IRQ info structure.
+* *flags* defines IRQ attributes:
+
+  * *VFIO_IRQ_INFO_EVENTFD* indicates the IRQ type can support server eventfd
+    signalling.
+  * *VFIO_IRQ_INFO_MASKABLE* indicates that the IRQ type supports the MASK and
+    UNMASK actions in a VFIO_USER_DEVICE_SET_IRQS message.
+  * *VFIO_IRQ_INFO_AUTOMASKED* indicates the IRQ type masks itself after being
+    triggered, and the client must send an UNMASK action to receive new
+    interrupts.
+  * *VFIO_IRQ_INFO_NORESIZE* indicates VFIO_USER_SET_IRQS operations setup
+    interrupts as a set, and new sub-indexes cannot be enabled without disabling
+    the entire type.
+
+* index is the index of IRQ type being queried, it is the only field that is
+  required to be set in the command message.
+* count describes the number of interrupts of the queried type.
+
+VFIO_USER_DEVICE_SET_IRQS
+-------------------------
+
+Message format
+^^^^^^^^^^^^^^
+
++--------------+------------------------+
+| Name         | Value                  |
++==============+========================+
+| Message ID   | <ID>                   |
++--------------+------------------------+
+| Command      | 7                      |
++--------------+------------------------+
+| Message size | 36 + any data          |
++--------------+------------------------+
+| Flags        | Reply bit set in reply |
++--------------+------------------------+
+| Error        | 0/errno                |
++--------------+------------------------+
+| IRQ set      | VFIO IRQ set           |
++--------------+------------------------+
+
+This command message is sent by the client to the server to set actions for
+device interrupt types. The VFIO IRQ set structure is defined in
+``<linux/vfio.h>`` (``struct vfio_irq_set``).
+
+VFIO IRQ set format
+^^^^^^^^^^^^^^^^^^^
+
++-------+--------+------------------------------+
+| Name  | Offset | Size                         |
++=======+========+==============================+
+| argsz | 16     | 4                            |
++-------+--------+------------------------------+
+| flags | 20     | 4                            |
++-------+--------+------------------------------+
+|       | +-----+-----------------------------+ |
+|       | | Bit | Definition                  | |
+|       | +=====+=============================+ |
+|       | | 0   | VFIO_IRQ_SET_DATA_NONE      | |
+|       | +-----+-----------------------------+ |
+|       | | 1   | VFIO_IRQ_SET_DATA_BOOL      | |
+|       | +-----+-----------------------------+ |
+|       | | 2   | VFIO_IRQ_SET_DATA_EVENTFD   | |
+|       | +-----+-----------------------------+ |
+|       | | 3   | VFIO_IRQ_SET_ACTION_MASK    | |
+|       | +-----+-----------------------------+ |
+|       | | 4   | VFIO_IRQ_SET_ACTION_UNMASK  | |
+|       | +-----+-----------------------------+ |
+|       | | 5   | VFIO_IRQ_SET_ACTION_TRIGGER | |
+|       | +-----+-----------------------------+ |
++-------+--------+------------------------------+
+| index | 24     | 4                            |
++-------+--------+------------------------------+
+| start | 28     | 4                            |
++-------+--------+------------------------------+
+| count | 32     | 4                            |
++-------+--------+------------------------------+
+| data  | 36     | variable                     |
++-------+--------+------------------------------+
+
+* *argsz* is the size of the VFIO IRQ set structure, including any *data* field.
+* *flags* defines the action performed on the interrupt range. The DATA flags
+  describe the data field sent in the message; the ACTION flags describe the
+  action to be performed. The flags are mutually exclusive for both sets.
+
+  * *VFIO_IRQ_SET_DATA_NONE* indicates there is no data field in the command.
+    The action is performed unconditionally.
+  * *VFIO_IRQ_SET_DATA_BOOL* indicates the data field is an array of boolean
+    bytes. The action is performed if the corresponding boolean is true.
+  * *VFIO_IRQ_SET_DATA_EVENTFD* indicates an array of event file descriptors
+    was sent in the message meta-data. These descriptors will be signalled when
+    the action defined by the action flags occurs. In AF_UNIX sockets, the
+    descriptors are sent as SCM_RIGHTS type ancillary data.
+  * *VFIO_IRQ_SET_ACTION_MASK* indicates a masking event. It can be used with
+    VFIO_IRQ_SET_DATA_BOOL or VFIO_IRQ_SET_DATA_NONE to mask an interrupt, or
+    with VFIO_IRQ_SET_DATA_EVENTFD to generate an event when the guest masks
+    the interrupt.
+  * *VFIO_IRQ_SET_ACTION_UNMASK* indicates an unmasking event. It can be used
+    with VFIO_IRQ_SET_DATA_BOOL or VFIO_IRQ_SET_DATA_NONE to unmask an
+    interrupt, or with VFIO_IRQ_SET_DATA_EVENTFD to generate an event when the
+    guest unmasks the interrupt.
+  * *VFIO_IRQ_SET_ACTION_TRIGGER* indicates a triggering event. It can be used
+    with VFIO_IRQ_SET_DATA_BOOL or VFIO_IRQ_SET_DATA_NONE to trigger an
+    interrupt, or with VFIO_IRQ_SET_DATA_EVENTFD to generate an event when the
+    server triggers the interrupt.
+
+* *index* is the index of IRQ type being setup.
+* *start* is the start of the sub-index being set.
+* *count* describes the number of sub-indexes being set. As a special case, a
+  count of 0 with data flags of VFIO_IRQ_SET_DATA_NONE disables all interrupts
+  of the index.
+* *data* is an optional field included when the
+  VFIO_IRQ_SET_DATA_BOOL flag is present. It contains an array of booleans
+  that specify whether the action is to be performed on the corresponding
+  index. It's used when the action is only performed on a subset of the range
+  specified.
+
+Not all interrupt types support every combination of data and action flags.
+The client must know the capabilities of the device and IRQ index before it
+sends a VFIO_USER_DEVICE_SET_IRQ message.
+
+.. _Read and Write Operations:
+
+Read and Write Operations
+-------------------------
+
+Not all I/O operations between the client and server can be done via direct
+access of memory mapped with an mmap() call. In these cases, the client and
+server use messages sent over the socket. It is expected that these operations
+will have lower performance than direct access.
+
+The client can access server memory with VFIO_USER_REGION_READ and
+VFIO_USER_REGION_WRITE commands. These share a common data structure that
+appears after the message header.
+
+REGION Read/Write Data
+^^^^^^^^^^^^^^^^^^^^^^
+
++--------+--------+----------+
+| Name   | Offset | Size     |
++========+========+==========+
+| Offset | 16     | 8        |
++--------+--------+----------+
+| Region | 24     | 4        |
++--------+--------+----------+
+| Count  | 28     | 4        |
++--------+--------+----------+
+| Data   | 32     | variable |
++--------+--------+----------+
+
+* *Offset* into the region being accessed.
+* *Region* is the index of the region being accessed.
+* *Count* is the size of the data to be transferred.
+* *Data* is the data to be read or written.
+
+The server can access client memory with VFIO_USER_DMA_READ and
+VFIO_USER_DMA_WRITE messages. These also share a common data structure that
+appears after the message header.
+
+DMA Read/Write Data
+^^^^^^^^^^^^^^^^^^^
+
++---------+--------+----------+
+| Name    | Offset | Size     |
++=========+========+==========+
+| Address | 16     | 8        |
++---------+--------+----------+
+| Count   | 24     | 4        |
++---------+--------+----------+
+| Data    | 28     | variable |
++---------+--------+----------+
+
+* *Address* is the area of client memory being accessed. This address must have
+  been previously exported to the server with a VFIO_USER_DMA_MAP message.
+* *Count* is the size of the data to be transferred.
+* *Data* is the data to be read or written.
+
+VFIO_USER_REGION_READ
+---------------------
+
+Message format
+^^^^^^^^^^^^^^
+
++--------------+------------------------+
+| Name         | Value                  |
++==============+========================+
+| Message ID   | <ID>                   |
++--------------+------------------------+
+| Command      | 8                      |
++--------------+------------------------+
+| Message size | 32 + data size         |
++--------------+------------------------+
+| Flags        | Reply bit set in reply |
++--------------+------------------------+
+| Error        | 0/errno                |
++--------------+------------------------+
+| Read info    | REGION read/write data |
++--------------+------------------------+
+
+This command message is sent from the client to the server to read from server
+memory.  In the command messages, there is no data, and the count is the amount
+of data to be read. The reply message must include the data read, and its count
+field is the amount of data read.
+
+VFIO_USER_REGION_WRITE
+----------------------
+
+Message format
+^^^^^^^^^^^^^^
+
++--------------+------------------------+
+| Name         | Value                  |
++==============+========================+
+| Message ID   | <ID>                   |
++--------------+------------------------+
+| Command      | 9                      |
++--------------+------------------------+
+| Message size | 32 + data size         |
++--------------+------------------------+
+| Flags        | Reply bit set in reply |
++--------------+------------------------+
+| Error        | 0/errno                |
++--------------+------------------------+
+| Write info   | REGION read/write data |
++--------------+------------------------+
+
+This command message is sent from the client to the server to write to server
+memory.  The command message must contain the data to be written, and its count
+field must contain the amount of write data. The count field in the reply
+message must be zero.
+
+VFIO_USER_DMA_READ
+------------------
+
+Message format
+^^^^^^^^^^^^^^
+
++--------------+------------------------+
+| Name         | Value                  |
++==============+========================+
+| Message ID   | <ID>                   |
++--------------+------------------------+
+| Command      | 10                     |
++--------------+------------------------+
+| Message size | 28 + data size         |
++--------------+------------------------+
+| Flags        | Reply bit set in reply |
++--------------+------------------------+
+| Error        | 0/errno                |
++--------------+------------------------+
+| DMA info     | DMA read/write data    |
++--------------+------------------------+
+
+This command message is sent from the server to the client to read from client
+memory.  In the command message, there is no data, and the count must will be
+the amount of data to be read. The reply message must include the data read,
+and its count field must be the amount of data read.
+
+VFIO_USER_DMA_WRITE
+-------------------
+
+Message format
+^^^^^^^^^^^^^^
+
++--------------+------------------------+
+| Name         | Value                  |
++==============+========================+
+| Message ID   | <ID>                   |
++--------------+------------------------+
+| Command      | 11                     |
++--------------+------------------------+
+| Message size | 28 + data size         |
++--------------+------------------------+
+| Flags        | Reply bit set in reply |
++--------------+------------------------+
+| Error        | 0/errno                |
++--------------+------------------------+
+| DMA info     | DMA read/write data    |
++--------------+------------------------+
+
+This command message is sent from the server to the client to write to server
+memory.  The command message must contain the data to be written, and its count
+field must contain the amount of write data. The count field in the reply
+message must be zero.
+
+VFIO_USER_VM_INTERRUPT
+----------------------
+
+Message format
+^^^^^^^^^^^^^^
+
++----------------+------------------------+
+| Name           | Value                  |
++================+========================+
+| Message ID     | <ID>                   |
++----------------+------------------------+
+| Command        | 12                     |
++----------------+------------------------+
+| Message size   | 24                     |
++----------------+------------------------+
+| Flags          | Reply bit set in reply |
++----------------+------------------------+
+| Error          | 0/errno                |
++----------------+------------------------+
+| Interrupt info | <interrupt>            |
++----------------+------------------------+
+
+This command message is sent from the server to the client to signal the device
+has raised an interrupt.
+
+Interrupt info format
+^^^^^^^^^^^^^^^^^^^^^
+
++-----------+--------+------+
+| Name      | Offset | Size |
++===========+========+======+
+| Index     | 16     | 4    |
++-----------+--------+------+
+| Sub-index | 20     | 4    |
++-----------+--------+------+
+
+* *Index* is the interrupt index; it is the same value used in
+  VFIO_USER_SET_IRQS.
+* *Sub-index* is relative to the index, e.g., the vector number used in PCI
+  MSI/X type interrupts.
+
+VFIO_USER_DEVICE_RESET
+----------------------
+
+Message format
+^^^^^^^^^^^^^^
+
++--------------+------------------------+
+| Name         | Value                  |
++==============+========================+
+| Message ID   | <ID>                   |
++--------------+------------------------+
+| Command      | 13                     |
++--------------+------------------------+
+| Message size | 16                     |
++--------------+------------------------+
+| Flags        | Reply bit set in reply |
++--------------+------------------------+
+| Error        | 0/errno                |
++--------------+------------------------+
+
+This command message is sent from the client to the server to reset the device.
+
+Appendices
+==========
+
+Unused VFIO ioctl() commands
+----------------------------
+
+The following VFIO commands do not have an equivalent vfio-user command:
+
+* VFIO_GET_API_VERSION
+* VFIO_CHECK_EXTENSION
+* VFIO_SET_IOMMU
+* VFIO_GROUP_GET_STATUS
+* VFIO_GROUP_SET_CONTAINER
+* VFIO_GROUP_UNSET_CONTAINER
+* VFIO_GROUP_GET_DEVICE_FD
+* VFIO_IOMMU_GET_INFO
+
+However, once support for live migration for VFIO devices is finalized some
+of the above commands may have to be handled by the client in their
+corresponding vfio-user form. This will be addressed in a future protocol
+version.
+
+VFIO groups and containers
+^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+The current VFIO implementation includes group and container idioms that
+describe how a device relates to the host IOMMU. In the vfio-user
+implementation, the IOMMU is implemented in SW by the client, and is not
+visible to the server. The simplest idea would be that the client put each
+device into its own group and container.
+
+Backend Program Conventions
+---------------------------
+
+vfio-user backend program conventions are based on the vhost-user ones.
+
+* The backend program must not daemonize itself.
+* No assumptions must be made as to what access the backend program has on the
+  system.
+* File descriptors 0, 1 and 2 must exist, must have regular
+  stdin/stdout/stderr semantics, and can be redirected.
+* The backend program must honor the SIGTERM signal.
+* The backend program must accept the following commands line options:
+
+  * ``--socket-path=PATH``: path to UNIX domain socket,
+  * ``--fd=FDNUM``: file descriptor for UNIX domain socket, incompatible with
+    ``--socket-path``
+* The backend program must be accompanied with a JSON file stored under
+  ``/usr/share/vfio-user``.