Re: [Qemu-devel] [PATCH v3 3/6] block QMP: Drop query-block member "type" (type= in info block)

[Markus Armbruster <armbru@redhat.com>]

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.