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

[Eduardo Habkost <ehabkost@redhat.com>]

On Fri, Sep 27, 2019 at 10:09:05AM +0100, Daniel P. Berrangé wrote:
> 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.

This is correct.  Only pconfig=off worked, and no accelerator
ever supported pconfig=on.

-- 
Eduardo