Re: [Qemu-devel] [PATCH 3/3] pc: Don't make CPU properties mandatory unless necessary

[Yash Mankad]

On 8/16/19 1:42 PM, Eduardo Habkost wrote:
> On Fri, Aug 16, 2019 at 02:22:58PM +0200, Markus Armbruster wrote:
>> Erik Skultety <address@hidden> writes:
>>
>>> On Fri, Aug 16, 2019 at 08:10:20AM +0200, Markus Armbruster wrote:
>>>> Eduardo Habkost <address@hidden> writes:
>>>>
>>>>> We have this issue reported when using libvirt to hotplug CPUs:
>>>>> https://bugzilla.redhat.com/show_bug.cgi?id=1741451
>>>>>
>>>>> Basically, libvirt is not copying die-id from
>>>>> query-hotpluggable-cpus, but die-id is now mandatory.
>>>> Uh-oh, "is now mandatory": making an optional property mandatory is an
>>>> incompatible change.  When did we do that?  Commit hash, please.
>>>>
>>>> [...]
>>>>
>>> I don't even see it as being optional ever - the property wasn't even
>>> recognized before commit 176d2cda0de introduced it as mandatory.
>> Compatibility break.
>>
>> Commit 176d2cda0de is in v4.1.0.  If I had learned about it a bit
>> earlier, I would've argued for a last minute fix or a revert.  Now we
>> have a regression in the release.
>>
>> Eduardo, I think this fix should go into v4.1.1.  Please add cc:
>> qemu-stable.
> I did it in v2.
>
>> How can we best avoid such compatibility breaks to slip in undetected?
>>
>> A static checker would be nice.  For vmstate, we have
>> scripts/vmstate-static-checker.py.  Not sure it's used.
> I don't think this specific bug would be detected with a static
> checker.  "die-id is mandatory" is not something that can be
> extracted by looking at QOM data structures.  The new rule was
> being enforced by the hotplug handler callbacks, and the hotplug
> handler call tree is a bit complex (too complex for my taste, but
> I digress).
>
> We could have detected this with a simple CPU hotplug automated
> test case, though.  Or with a very simple -device test case like
> the one I have submitted with this patch.
>
> This was detected by libvirt automated test cases.  It would be
> nice if this was run during the -rc stage and not only after the
> 4.1.0 release, though.
>
> I don't know details of the test job.  Danilo, Mirek, Yash: do
> you know how this bug was detected, and what we could do to run
> the same test jobs in upstream QEMU release candidates?

This bug was caught by our internal gating tests.

The libvirt gating tests for the virt module include the
following Avocado-VT test case:

libvirt_vcpu_plug_unplug.positive_test.vcpu_set.live.vm_operate.save

This job failed with the error that you can see in the description
of the BZ#1741451 [0].

If you think that this would have been caught by a simple hotplug
case, I'd recommend adding a test for hotplug to avocado_qemu.
Otherwise, if you want, I can look into adding this particular
libvirt test case to our QEMU CI efforts.

Thanks,
Yash

[0] https://bugzilla.redhat.com/show_bug.cgi?id=1741451#c0



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