| Message ID | 1305212715-26767-4-git-send-email-armbru@redhat.com |
|---|---|
| State | New |
| Headers | show |
On Thu, 12 May 2011 17:05:12 +0200 Markus Armbruster <armbru@redhat.com> wrote: > Its value is unreliable: a block device used as floppy has type > "floppy" if created with if=floppy, but type "hd" if created with > if=none. > > That's because with if=none, the type is at best a declaration of > intent: the drive can be connected to any guest device. Its type is > really the guest device's business. Reporting it here is wrong. It reports how the guest is using the device, right? I'd say that's what users/clients are interested in knowing. Also, we can't just drop it from QMP. We should first note it's deprecated.
Luiz Capitulino <lcapitulino@redhat.com> writes: > On Thu, 12 May 2011 17:05:12 +0200 > Markus Armbruster <armbru@redhat.com> wrote: > >> Its value is unreliable: a block device used as floppy has type >> "floppy" if created with if=floppy, but type "hd" if created with >> if=none. >> >> That's because with if=none, the type is at best a declaration of >> intent: the drive can be connected to any guest device. Its type is >> really the guest device's business. Reporting it here is wrong. > > It reports how the guest is using the device, right? I'd say that's what > users/clients are interested in knowing. The value is *unreliable*. It may or may not match how the guest is using the device. I doubt users are interested in unreliable information. > Also, we can't just drop it from QMP. We should first note it's deprecated. Would you accept a change to the more honest value "unknown" for the deprecation period?
On Thu, 12 May 2011 19:12:56 +0200 Markus Armbruster <armbru@redhat.com> wrote: > Luiz Capitulino <lcapitulino@redhat.com> writes: > > > On Thu, 12 May 2011 17:05:12 +0200 > > Markus Armbruster <armbru@redhat.com> wrote: > > > >> Its value is unreliable: a block device used as floppy has type > >> "floppy" if created with if=floppy, but type "hd" if created with > >> if=none. > >> > >> That's because with if=none, the type is at best a declaration of > >> intent: the drive can be connected to any guest device. Its type is > >> really the guest device's business. Reporting it here is wrong. > > > > It reports how the guest is using the device, right? I'd say that's what > > users/clients are interested in knowing. > > The value is *unreliable*. It may or may not match how the guest is > using the device. I doubt users are interested in unreliable > information. Can't it be fixed? And how are users/clients supposed to find out how the guest is using its block devices? > > Also, we can't just drop it from QMP. We should first note it's deprecated. > > Would you accept a change to the more honest value "unknown" for the > deprecation period? We have to avoid breaking the protocol. Changing something that has always been reported as 'cdrom' to 'unknown' will likely cause as many as damages as dropping the command. The best solution I can think of is noting in the documentation that the information is unreliable and explain what clients interested in knowing this info should do.
Luiz Capitulino <lcapitulino@redhat.com> writes: > On Thu, 12 May 2011 19:12:56 +0200 > Markus Armbruster <armbru@redhat.com> wrote: > >> Luiz Capitulino <lcapitulino@redhat.com> writes: >> >> > On Thu, 12 May 2011 17:05:12 +0200 >> > Markus Armbruster <armbru@redhat.com> wrote: >> > >> >> Its value is unreliable: a block device used as floppy has type >> >> "floppy" if created with if=floppy, but type "hd" if created with >> >> if=none. >> >> >> >> That's because with if=none, the type is at best a declaration of >> >> intent: the drive can be connected to any guest device. Its type is >> >> really the guest device's business. Reporting it here is wrong. >> > >> > It reports how the guest is using the device, right? I'd say that's what >> > users/clients are interested in knowing. >> >> The value is *unreliable*. It may or may not match how the guest is >> using the device. I doubt users are interested in unreliable >> information. > > Can't it be fixed? And how are users/clients supposed to find out how > the guest is using its block devices? To find out more about the guest's devices, examine the guest's devices: info qtree. You don't expect to find the guest serial devices in in "info chardev", either. query-block's type member is a mistake, because it mixes up guest device info with the host device info. Dropping it is a bug fix. The fact that its value is unreliable is merely icing on the cake. >> > Also, we can't just drop it from QMP. We should first note it's deprecated. >> >> Would you accept a change to the more honest value "unknown" for the >> deprecation period? > > We have to avoid breaking the protocol. Changing something that has always > been reported as 'cdrom' to 'unknown' will likely cause as many as damages > as dropping the command. I can cause damage only if somebody is using it. Which I doubt. Remember, the value is unreliable. It's a *lie*. We can stop lying in two ways: shut up (drop member "type"), or tell the truth (change the value to "unknown", which is a documented value of "type"). > The best solution I can think of is noting in the documentation that the > information is unreliable and explain what clients interested in knowing > this info should do. I'd be much more willing to jump through compatibility hoops if there was *one* known user of this particular detail of QMP. But if you insist on us continuing to lie, I'll find a way to continue to lie. I'm resisting it, because I think it's a disservice to our users.
On Thu, 12 May 2011 19:54:40 +0200 Markus Armbruster <armbru@redhat.com> wrote: > Luiz Capitulino <lcapitulino@redhat.com> writes: > > > On Thu, 12 May 2011 19:12:56 +0200 > > Markus Armbruster <armbru@redhat.com> wrote: > > > >> Luiz Capitulino <lcapitulino@redhat.com> writes: > >> > >> > On Thu, 12 May 2011 17:05:12 +0200 > >> > Markus Armbruster <armbru@redhat.com> wrote: > >> > > >> >> Its value is unreliable: a block device used as floppy has type > >> >> "floppy" if created with if=floppy, but type "hd" if created with > >> >> if=none. > >> >> > >> >> That's because with if=none, the type is at best a declaration of > >> >> intent: the drive can be connected to any guest device. Its type is > >> >> really the guest device's business. Reporting it here is wrong. > >> > > >> > It reports how the guest is using the device, right? I'd say that's what > >> > users/clients are interested in knowing. > >> > >> The value is *unreliable*. It may or may not match how the guest is > >> using the device. I doubt users are interested in unreliable > >> information. > > > > Can't it be fixed? And how are users/clients supposed to find out how > > the guest is using its block devices? > > To find out more about the guest's devices, examine the guest's devices: > info qtree. Which is not converted to QMP yet. > You don't expect to find the guest serial devices in in "info chardev", > either. query-block's type member is a mistake, because it mixes up > guest device info with the host device info. Dropping it is a bug fix. I understand why you're dropping it, what I don't want to do is to break working clients. For example, there might be clients out there using it in a way that it's expected to work (eg. if=floppy). Of course that I'm assuming that such a client exist. > The fact that its value is unreliable is merely icing on the cake. > > >> > Also, we can't just drop it from QMP. We should first note it's deprecated. > >> > >> Would you accept a change to the more honest value "unknown" for the > >> deprecation period? > > > > We have to avoid breaking the protocol. Changing something that has always > > been reported as 'cdrom' to 'unknown' will likely cause as many as damages > > as dropping the command. > > I can cause damage only if somebody is using it. Which I doubt. Me too and I'd agree with this patch if I was 100% sure. But it's impossible to be sure, unless we do it by trial and error which is harmful. > Remember, the value is unreliable. It's a *lie*. We can stop lying in > two ways: shut up (drop member "type"), or tell the truth (change the > value to "unknown", which is a documented value of "type"). Can we set it to 'unknown' when if=none? > > The best solution I can think of is noting in the documentation that the > > information is unreliable and explain what clients interested in knowing > > this info should do. > > I'd be much more willing to jump through compatibility hoops if there > was *one* known user of this particular detail of QMP. Ideally, yes, but it's the type of thing we'll never know. > But if you insist on us continuing to lie, I'll find a way to continue > to lie. I'm resisting it, because I think it's a disservice to our > users. I also want to do the best for our users and I don't want to ignore the bug, but we don't know whether there are clients using the field. If we drop it and a client does use it, then we'll have failed in doing the best.
Luiz Capitulino <lcapitulino@redhat.com> writes: > On Thu, 12 May 2011 19:54:40 +0200 > Markus Armbruster <armbru@redhat.com> wrote: > >> Luiz Capitulino <lcapitulino@redhat.com> writes: >> >> > On Thu, 12 May 2011 19:12:56 +0200 >> > Markus Armbruster <armbru@redhat.com> wrote: >> > >> >> Luiz Capitulino <lcapitulino@redhat.com> writes: >> >> >> >> > On Thu, 12 May 2011 17:05:12 +0200 >> >> > Markus Armbruster <armbru@redhat.com> wrote: >> >> > >> >> >> Its value is unreliable: a block device used as floppy has type >> >> >> "floppy" if created with if=floppy, but type "hd" if created with >> >> >> if=none. >> >> >> >> >> >> That's because with if=none, the type is at best a declaration of >> >> >> intent: the drive can be connected to any guest device. Its type is >> >> >> really the guest device's business. Reporting it here is wrong. >> >> > >> >> > It reports how the guest is using the device, right? I'd say that's what >> >> > users/clients are interested in knowing. >> >> >> >> The value is *unreliable*. It may or may not match how the guest is >> >> using the device. I doubt users are interested in unreliable >> >> information. >> > >> > Can't it be fixed? And how are users/clients supposed to find out how >> > the guest is using its block devices? >> >> To find out more about the guest's devices, examine the guest's devices: >> info qtree. > > Which is not converted to QMP yet. It's where the information is. No use looking for it where it isn't. If users need it, we better convert info qtree. >> You don't expect to find the guest serial devices in in "info chardev", >> either. query-block's type member is a mistake, because it mixes up >> guest device info with the host device info. Dropping it is a bug fix. > > I understand why you're dropping it, what I don't want to do is to break > working clients. For example, there might be clients out there using it > in a way that it's expected to work (eg. if=floppy). > > Of course that I'm assuming that such a client exist. A client new enough to use QMP, old enough to use legacy if=floppy, silly enough to query for information it already knows (where did I put that drive again?), and brittle enough to break hard when it gets no type or type "unknown". >> The fact that its value is unreliable is merely icing on the cake. >> >> >> > Also, we can't just drop it from QMP. We should first note it's deprecated. >> >> >> >> Would you accept a change to the more honest value "unknown" for the >> >> deprecation period? >> > >> > We have to avoid breaking the protocol. Changing something that has always >> > been reported as 'cdrom' to 'unknown' will likely cause as many as damages >> > as dropping the command. >> >> I can cause damage only if somebody is using it. Which I doubt. > > Me too and I'd agree with this patch if I was 100% sure. But it's impossible > to be sure, unless we do it by trial and error which is harmful. > >> Remember, the value is unreliable. It's a *lie*. We can stop lying in >> two ways: shut up (drop member "type"), or tell the truth (change the >> value to "unknown", which is a documented value of "type"). > > Can we set it to 'unknown' when if=none? Maybe. Makes query-block mix up host and guest information again. Purging guest information from block.c is the point of this series. Therefore, query-block can't be done in block.c anymore. It needs to move to blockdev.c, where the mixed-up-by-design DriveInfo is available. It could move back when we finally clean up query-block. Even with such compatibility gymnastics, it could still break your hypothetical client. >> > The best solution I can think of is noting in the documentation that the >> > information is unreliable and explain what clients interested in knowing >> > this info should do. >> >> I'd be much more willing to jump through compatibility hoops if there >> was *one* known user of this particular detail of QMP. > > Ideally, yes, but it's the type of thing we'll never know. > >> But if you insist on us continuing to lie, I'll find a way to continue >> to lie. I'm resisting it, because I think it's a disservice to our >> users. > > I also want to do the best for our users and I don't want to ignore the bug, > but we don't know whether there are clients using the field. If we drop it > and a client does use it, then we'll have failed in doing the best. Good enough is good enough.
On Fri, 13 May 2011 10:26:38 +0200 Markus Armbruster <armbru@redhat.com> wrote: > Luiz Capitulino <lcapitulino@redhat.com> writes: > > > On Thu, 12 May 2011 19:54:40 +0200 > > Markus Armbruster <armbru@redhat.com> wrote: > > > >> Luiz Capitulino <lcapitulino@redhat.com> writes: > >> > >> > On Thu, 12 May 2011 19:12:56 +0200 > >> > Markus Armbruster <armbru@redhat.com> wrote: > >> > > >> >> Luiz Capitulino <lcapitulino@redhat.com> writes: > >> >> > >> >> > On Thu, 12 May 2011 17:05:12 +0200 > >> >> > Markus Armbruster <armbru@redhat.com> wrote: > >> >> > > >> >> >> Its value is unreliable: a block device used as floppy has type > >> >> >> "floppy" if created with if=floppy, but type "hd" if created with > >> >> >> if=none. > >> >> >> > >> >> >> That's because with if=none, the type is at best a declaration of > >> >> >> intent: the drive can be connected to any guest device. Its type is > >> >> >> really the guest device's business. Reporting it here is wrong. > >> >> > > >> >> > It reports how the guest is using the device, right? I'd say that's what > >> >> > users/clients are interested in knowing. > >> >> > >> >> The value is *unreliable*. It may or may not match how the guest is > >> >> using the device. I doubt users are interested in unreliable > >> >> information. > >> > > >> > Can't it be fixed? And how are users/clients supposed to find out how > >> > the guest is using its block devices? > >> > >> To find out more about the guest's devices, examine the guest's devices: > >> info qtree. > > > > Which is not converted to QMP yet. > > It's where the information is. No use looking for it where it isn't. > If users need it, we better convert info qtree. Sure, we should do that. > >> You don't expect to find the guest serial devices in in "info chardev", > >> either. query-block's type member is a mistake, because it mixes up > >> guest device info with the host device info. Dropping it is a bug fix. > > > > I understand why you're dropping it, what I don't want to do is to break > > working clients. For example, there might be clients out there using it > > in a way that it's expected to work (eg. if=floppy). > > > > Of course that I'm assuming that such a client exist. > > A client new enough to use QMP, old enough to use legacy if=floppy, Legacy option != unsupported option. I wouldn't blame anyone for doing weird mixtures with our options, as our external interfaces are not easy to use. And that's *our* problem, not user's. If the user did something stupid, it probably means *we* did something stupid. One could argue that this is an argument in favor of this change, and I'd promptly agree, as my main point is not about not dropping it, but respecting our deprecation policy. > silly enough to query for information it already knows (where did I put > that drive again?), This assumes the client itself has started the VM, or it knows the VM beforehand. What if it doesn't? Don't we support connecting to an already started VM then? > and brittle enough to break hard when it gets no > type or type "unknown". I said that changing something that always reported 'cdrom' to 'unknown' is likely to cause damage. I can't tell whether it's going to break hard or not. > >> The fact that its value is unreliable is merely icing on the cake. > >> > >> >> > Also, we can't just drop it from QMP. We should first note it's deprecated. > >> >> > >> >> Would you accept a change to the more honest value "unknown" for the > >> >> deprecation period? > >> > > >> > We have to avoid breaking the protocol. Changing something that has always > >> > been reported as 'cdrom' to 'unknown' will likely cause as many as damages > >> > as dropping the command. > >> > >> I can cause damage only if somebody is using it. Which I doubt. > > > > Me too and I'd agree with this patch if I was 100% sure. But it's impossible > > to be sure, unless we do it by trial and error which is harmful. > > > >> Remember, the value is unreliable. It's a *lie*. We can stop lying in > >> two ways: shut up (drop member "type"), or tell the truth (change the > >> value to "unknown", which is a documented value of "type"). > > > > Can we set it to 'unknown' when if=none? > > Maybe. Makes query-block mix up host and guest information again. It's temporary, just to respect our deprecation policy and give us time to provide a viable alternative. > Purging guest information from block.c is the point of this series. > Therefore, query-block can't be done in block.c anymore. It needs to > move to blockdev.c, where the mixed-up-by-design DriveInfo is available. > It could move back when we finally clean up query-block. > > Even with such compatibility gymnastics, it could still break your > hypothetical client. Our deprecation policy is not hypothetical. There isn't case by case. Either we respect it or we don't. > >> > The best solution I can think of is noting in the documentation that the > >> > information is unreliable and explain what clients interested in knowing > >> > this info should do. > >> > >> I'd be much more willing to jump through compatibility hoops if there > >> was *one* known user of this particular detail of QMP. > > > > Ideally, yes, but it's the type of thing we'll never know. > > > >> But if you insist on us continuing to lie, I'll find a way to continue > >> to lie. I'm resisting it, because I think it's a disservice to our > >> users. > > > > I also want to do the best for our users and I don't want to ignore the bug, > > but we don't know whether there are clients using the field. If we drop it > > and a client does use it, then we'll have failed in doing the best. > > Good enough is good enough. Breaking our deprecation policy is not what I'd call good enough.
Luiz Capitulino <lcapitulino@redhat.com> writes: > On Fri, 13 May 2011 10:26:38 +0200 > Markus Armbruster <armbru@redhat.com> wrote: > >> Luiz Capitulino <lcapitulino@redhat.com> writes: >> >> > On Thu, 12 May 2011 19:54:40 +0200 >> > Markus Armbruster <armbru@redhat.com> wrote: >> > >> >> Luiz Capitulino <lcapitulino@redhat.com> writes: >> >> >> >> > On Thu, 12 May 2011 19:12:56 +0200 >> >> > Markus Armbruster <armbru@redhat.com> wrote: >> >> > >> >> >> Luiz Capitulino <lcapitulino@redhat.com> writes: [...] >> >> >> > Also, we can't just drop it from QMP. We should first note it's deprecated. >> >> >> >> >> >> Would you accept a change to the more honest value "unknown" for the >> >> >> deprecation period? >> >> > >> >> > We have to avoid breaking the protocol. Changing something that has always >> >> > been reported as 'cdrom' to 'unknown' will likely cause as many as damages >> >> > as dropping the command. >> >> >> >> I can cause damage only if somebody is using it. Which I doubt. >> > >> > Me too and I'd agree with this patch if I was 100% sure. But it's impossible >> > to be sure, unless we do it by trial and error which is harmful. >> > >> >> Remember, the value is unreliable. It's a *lie*. We can stop lying in >> >> two ways: shut up (drop member "type"), or tell the truth (change the >> >> value to "unknown", which is a documented value of "type"). >> > >> > Can we set it to 'unknown' when if=none? >> >> Maybe. Makes query-block mix up host and guest information again. > > It's temporary, just to respect our deprecation policy and give us time > to provide a viable alternative. > >> Purging guest information from block.c is the point of this series. >> Therefore, query-block can't be done in block.c anymore. It needs to >> move to blockdev.c, where the mixed-up-by-design DriveInfo is available. >> It could move back when we finally clean up query-block. >> >> Even with such compatibility gymnastics, it could still break your >> hypothetical client. > > Our deprecation policy is not hypothetical. There isn't case by case. Either > we respect it or we don't. My point is: even your deprecation policy cannot protect your hypothetical client. The only way to ensure not even the silliest hypothetical client breaks is not to change anything. Which means we cannot fix query-block not to lie about the type, period. Which means we have to deprecate query-block wholesale to fix that bug. Kevin, please apply patches 1+2+6. Feel free to drop 3-5. [...]
On Fri, 13 May 2011 16:13:55 +0200 Markus Armbruster <armbru@redhat.com> wrote: > Luiz Capitulino <lcapitulino@redhat.com> writes: > > > On Fri, 13 May 2011 10:26:38 +0200 > > Markus Armbruster <armbru@redhat.com> wrote: > > > >> Luiz Capitulino <lcapitulino@redhat.com> writes: > >> > >> > On Thu, 12 May 2011 19:54:40 +0200 > >> > Markus Armbruster <armbru@redhat.com> wrote: > >> > > >> >> Luiz Capitulino <lcapitulino@redhat.com> writes: > >> >> > >> >> > On Thu, 12 May 2011 19:12:56 +0200 > >> >> > Markus Armbruster <armbru@redhat.com> wrote: > >> >> > > >> >> >> Luiz Capitulino <lcapitulino@redhat.com> writes: > [...] > >> >> >> > Also, we can't just drop it from QMP. We should first note it's deprecated. > >> >> >> > >> >> >> Would you accept a change to the more honest value "unknown" for the > >> >> >> deprecation period? > >> >> > > >> >> > We have to avoid breaking the protocol. Changing something that has always > >> >> > been reported as 'cdrom' to 'unknown' will likely cause as many as damages > >> >> > as dropping the command. > >> >> > >> >> I can cause damage only if somebody is using it. Which I doubt. > >> > > >> > Me too and I'd agree with this patch if I was 100% sure. But it's impossible > >> > to be sure, unless we do it by trial and error which is harmful. > >> > > >> >> Remember, the value is unreliable. It's a *lie*. We can stop lying in > >> >> two ways: shut up (drop member "type"), or tell the truth (change the > >> >> value to "unknown", which is a documented value of "type"). > >> > > >> > Can we set it to 'unknown' when if=none? > >> > >> Maybe. Makes query-block mix up host and guest information again. > > > > It's temporary, just to respect our deprecation policy and give us time > > to provide a viable alternative. > > > >> Purging guest information from block.c is the point of this series. > >> Therefore, query-block can't be done in block.c anymore. It needs to > >> move to blockdev.c, where the mixed-up-by-design DriveInfo is available. > >> It could move back when we finally clean up query-block. > >> > >> Even with such compatibility gymnastics, it could still break your > >> hypothetical client. > > > > Our deprecation policy is not hypothetical. There isn't case by case. Either > > we respect it or we don't. > > My point is: even your deprecation policy cannot protect your > hypothetical client. > > The only way to ensure not even the silliest hypothetical client breaks > is not to change anything. Which means we cannot fix query-block not to > lie about the type, period. Which means we have to deprecate > query-block wholesale to fix that bug. Why don't we force the known broken cases (like if=none) to 'unknown', note in the doc that the field is unreliable and specify that it can be dropped in 2 or 3 releases? > > Kevin, please apply patches 1+2+6. Feel free to drop 3-5. > > [...] >
diff --git a/block.c b/block.c index f731c7a..c6dc42a 100644 --- a/block.c +++ b/block.c @@ -1704,9 +1704,8 @@ static void bdrv_print_dict(QObject *obj, void *opaque) bs_dict = qobject_to_qdict(obj); - monitor_printf(mon, "%s: type=%s removable=%d", + monitor_printf(mon, "%s: removable=%d", qdict_get_str(bs_dict, "device"), - qdict_get_str(bs_dict, "type"), qdict_get_bool(bs_dict, "removable")); if (qdict_get_bool(bs_dict, "removable")) { @@ -1747,23 +1746,10 @@ void bdrv_info(Monitor *mon, QObject **ret_data) QTAILQ_FOREACH(bs, &bdrv_states, list) { QObject *bs_obj; - const char *type = "unknown"; - - switch(bs->type) { - case BDRV_TYPE_HD: - type = "hd"; - break; - case BDRV_TYPE_CDROM: - type = "cdrom"; - break; - case BDRV_TYPE_FLOPPY: - type = "floppy"; - break; - } - bs_obj = qobject_from_jsonf("{ 'device': %s, 'type': %s, " + bs_obj = qobject_from_jsonf("{ 'device': %s, " "'removable': %i, 'locked': %i }", - bs->device_name, type, bs->removable, + bs->device_name, bs->removable, bs->locked); if (bs->drv) { diff --git a/qmp-commands.hx b/qmp-commands.hx index fbd98ee..b1c8206 100644 --- a/qmp-commands.hx +++ b/qmp-commands.hx @@ -1038,8 +1038,6 @@ is a json-array of all devices. Each json-object contain the following: - "device": device name (json-string) -- "type": device type (json-string) - - Possible values: "hd", "cdrom", "floppy", "unknown" - "removable": true if the device is removable, false otherwise (json-bool) - "locked": true if the device is locked, false otherwise (json-bool) - "inserted": only present if the device is inserted, it is a json-object @@ -1070,25 +1068,21 @@ Example: "encrypted":false, "file":"disks/test.img" }, - "type":"hd" }, { "device":"ide1-cd0", "locked":false, "removable":true, - "type":"cdrom" }, { "device":"floppy0", "locked":false, "removable":true, - "type": "floppy" }, { "device":"sd0", "locked":false, "removable":true, - "type":"floppy" } ] }
Its value is unreliable: a block device used as floppy has type "floppy" if created with if=floppy, but type "hd" if created with if=none. That's because with if=none, the type is at best a declaration of intent: the drive can be connected to any guest device. Its type is really the guest device's business. Reporting it here is wrong. Signed-off-by: Markus Armbruster <armbru@redhat.com> --- block.c | 20 +++----------------- qmp-commands.hx | 6 ------ 2 files changed, 3 insertions(+), 23 deletions(-)