Re: [PATCH] KVM: x86: Advertise AMX-COMPLEX CPUID to userspace

[Sean Christopherson <seanjc@google.com>]

On Wed, Aug 02, 2023, Tao Su wrote:
> Latest Intel platform GraniteRapids-D introduces AMX-COMPLEX, which adds
> two instructions to perform matrix multiplication of two tiles containing
> complex elements and accumulate the results into a packed single precision
> tile.
> 
> AMX-COMPLEX is enumerated via CPUID.(EAX=7,ECX=1):EDX[bit 8]
> 
> Since there are no new VMX controls or additional host enabling required
> for guests to use this feature, advertise the CPUID to userspace.

Nit, I would rather justify this (last paragraph) with something like:

  Advertise AMX_COMPLEX if it's supported in hardware.  There are no VMX
  controls for the feature, i.e. the instructions can't be interecepted, and
  KVM advertises base AMX in CPUID if AMX is supported in hardware, even if
  KVM doesn't advertise AMX as being supported in XCR0, e.g. because the
  process didn't opt-in to allocating tile data.

If the above is accurate and there are no objections, I'll fixup the changelog
when applying.

Side topic, this does make me wonder if advertising AMX when XTILE_DATA isn't
permitted is a bad idea.  But no one has complained, and chasing down all the
dependent AMX features would get annoying, so I'm inclined to keep the status quo.