Re: [Qemu-devel] [PATCH] target-i386: add feature flags for CPUID[EAX=0xd, ECX=1]

[Eduardo Habkost]

On Wed, Nov 26, 2014 at 10:20:12AM +0100, Paolo Bonzini wrote:
> 
> 
> On 25/11/2014 21:02, Paolo Bonzini wrote:
> > > > +static const char *cpuid_xsave_feature_name[] = {
> > > > +    "xsaveopt", "xsavec", "xgetbv1", "xsaves",
> > > 
> > > None of the above features introduce any new state that might need to be
> > > migrated, or will require other changes in QEMU to work, right?
> > > 
> > > It looks like they don't introduce any extra state, but if they do, they
> > > need to be added to unmigratable_flags until migration support is
> > > implemented.
> > > 
> > > If they require other QEMU changes, it would be nice if KVM reported
> > > them using KVM_CHECK_EXTENSION instead of GET_SUPPORTED_CPUID, so it
> > > wouldn't break "-cpu host".
> > 
> > No, they don't.
> 
> Actually, xsaves does but I don't think KVM_CHECK_EXTENSION is right.
> It's just another MSR, and we haven't used KVM_CHECK_EXTENSION for new
> MSRs and new XSAVE areas (last example: avx512).

If the changes needed are only to support migration (this is the case if
it's just another MSR handled by KVM, or additional XSAVE areas),
GET_SUPPORTED_CPUID is still reasonable, because features that are
unknown to QEMU are always considered unmigratable until we add the
feature name to the feature_name arrays. (That's why we need to know if
the feature introduces additional state when adding the feature names to
the array.)

If other changes are required to make the feature work even if no
migration is required, then adding them to GET_SUPPORTED_CPUID would
break "-cpu host" on older QEMUs. I don't think that's the case here,
but I wanted to confirm.

(Should we add those observations to Documentation/virtual/kvm/api.txt?)


> 
> Since no hardware really exists for it, and KVM does not support it
> anyway, I think it's simplest to leave xsaves out for now.  Is this right?

If unsure, it won't hurt to add the feature to unmigratable_features by
now. Making QEMU aware of the feature name is still useful.

-- 
Eduardo