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

[Eduardo Habkost <ehabkost@redhat.com>]

On Wed, Jun 13, 2018 at 06:21:58PM +0000, Moger, Babu wrote:
> 
> 
> > -----Original Message-----
> > From: Eduardo Habkost [mailto:ehabkost@redhat.com]
> > Sent: Wednesday, June 13, 2018 1:18 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 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.
> > 
> 
> Ok Sure.

Sorry, I forgot we still need to decide what to do if TOPOEXT is
enabled in the CPU model (or command-line) but the -smp options
are not compatible with it.

In other words, what should guest see on CPUID if using:

"-machine pc-q35-3.0 -cpu EPYC -smp 64,cores=64"
or:
"-machine pc-q35-3.0 -cpu Opteron_G5,+topoext -smp 64,cores=64"

I wonder what would happen if we just return zeroes on
CPUID[0x800001E] if !topology_supports_topoext(), instead of
trying to clear/set TOPOEXT depending on the -smp option?  It
would make things much simpler for QEMU and libvirt.

-- 
Eduardo