Re: [Qemu-devel] [PATCH v13 3/5] i386: Enable TOPOEXT feature on AMD EPYC CPU

[Eduardo Habkost <ehabkost@redhat.com>]

On Wed, Jun 13, 2018 at 06:10:30PM +0000, Moger, Babu wrote:
> > -----Original Message-----
> > From: Eduardo Habkost [mailto:ehabkost@redhat.com]
> > Sent: Wednesday, June 13, 2018 12:18 PM
> > To: Moger, Babu <Babu.Moger@amd.com>
> > Cc: mst@redhat.com; marcel.apfelbaum@gmail.com; pbonzini@redhat.com;
> > rth@twiddle.net; mtosatti@redhat.com; qemu-devel@nongnu.org;
> > kvm@vger.kernel.org; kash@tripleback.net; geoff@hostfission.com; Jiri
> > Denemark <jdenemar@redhat.com>
> > Subject: Re: [PATCH v13 3/5] i386: Enable TOPOEXT feature on AMD EPYC
> > CPU
> > 
> > On Wed, Jun 13, 2018 at 04:52:18PM +0000, Moger, Babu wrote:
> > [...]
> > > > What do you think our options are here?
> > >
> > > Should we drop automatic topoext completely and move forward?
> > > What are your thoughts?
> > 
> > Let's drop automatic topoext by now, and see if we find solutions
> > later.  I don't want to hold the rest of the patches because of
> > this.
> 
> Ok. I will drop topoext.
> 
> > 
> > I'm thinking we could simply make kvm_arch_get_supported_cpuid()
> > always return TOPOEXT on AMD CPUs, because the feature flag don't
> > really depend on any KVM code to work (is that correct?).
> 
> Yes, that is correct. I don't see any dependent code on TOPOEXT in KVM driver.
> 
> Ok. Let me add TOPOEXT flag for all the AMD cpus and see how it goes.

Hmm, this could actually solve all of our problems, then:

We can forget about auto-topoext: just add TOPOEXT in
kvm_arch_get_supported_cpuid(), add TOPOEXT unconditionally to
the CPU models where you are interested into (EPYC only?), and
add topoext=off to pc-2.12 compat_props.

Sorry for not noticing that before.  I was incorrectly assuming
that TOPOEXT was safe to enable only if it was returned by
GET_SUPPORTED_CPUID.

-- 
Eduardo