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

[Markus Armbruster <armbru@redhat.com>]

Erik Skultety <eskultet@redhat.com> writes:

> On Fri, Aug 16, 2019 at 08:10:20AM +0200, Markus Armbruster wrote:
>> Eduardo Habkost <ehabkost@redhat.com> 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.

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.