Re: [PATCH] i386: Re-add "pconfig" CPUID flag name

[Daniel P . Berrangé]

On Thu, Sep 26, 2019 at 09:42:20PM -0300, Eduardo Habkost wrote:
> (CCing libvir-list)
> 
> On Thu, Sep 26, 2019 at 11:58:30PM +0200, Paolo Bonzini wrote:
> > Is this really needed? QEMU's value of pconfig=on vs. off should be
> > provided by QMP CPU model queries, if a property is not available then
> > Libvirt should not try to set it to off.
> > 
> 
> Libvirt can easily work around it for new VMs, and it should.
> 
> The issue are VMs that were created with QEMU 3.1.0.  QEMU 3.1.0
> was telling libvirt "Icelake-Server can't be used unless
> pconfig=off is used", and libvirt was adding pconfig=off to the
> domain XML as expected.
> 
> It would be wrong for libvirt to remove a device option when
> migrating an existing VM to another QEMU version.  We can change
> the rules (and document that), but do we want to?

IIUC currently any existing VMs with Icelake-Server will *not* have
pconfig present, since libvirt is going to set pconfig=off

QEMU has now dropped pconfig CPUID entirely. If libvirt were to stop
setting pconfig=off, then there is NO guest ABI change, so we'd be
safe in that sense.

I'll defer to Jiri on how we'd actually go about making that work
in libvirt from a code POV.


Regards,
Daniel
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