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

From: Eduardo Habkost <ehabkost@redhat.com>
To: Markus Armbruster <armbru@redhat.com>
Cc: Yash Mankad <ymankad@redhat.com>,
	Peter Krempa <pkrempa@redhat.com>,
	Like Xu <like.xu@linux.intel.com>,
	"Michael S. Tsirkin" <mst@redhat.com>,
	Erik Skultety <eskultet@redhat.com>,
	qemu-devel@nongnu.org, Paolo Bonzini <pbonzini@redhat.com>,
	Igor Mammedov <imammedo@redhat.com>,
	Miroslav Rezanina <mrezanin@redhat.com>,
	"Danilo C. L. de Paula" <ddepaula@redhat.com>,
	Richard Henderson <rth@twiddle.net>
Subject: Re: [Qemu-devel] [PATCH 3/3] pc: Don't make CPU properties mandatory unless necessary
Date: Fri, 16 Aug 2019 14:42:41 -0300	[thread overview]
Message-ID: <20190816174241.GE3908@habkost.net> (raw)
In-Reply-To: <87h86hpae5.fsf@dusky.pond.sub.org>

On Fri, Aug 16, 2019 at 02:22:58PM +0200, Markus Armbruster wrote:
> 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.

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?

-- 
Eduardo