Re: [PATCH v6 12/25] KVM: x86: Refresh CPUID on write to guest MSR_IA32_XSS

[Sean Christopherson]

On Tue, Oct 31, 2023, Maxim Levitsky wrote:
> On Thu, 2023-09-14 at 02:33 -0400, Yang Weijiang wrote:
> > @@ -312,6 +313,17 @@ static u64 vcpu_get_supported_xcr0(struct kvm_vcpu *vcpu)
> >  	return (best->eax | ((u64)best->edx << 32)) & kvm_caps.supported_xcr0;
> >  }
> >  
> > +static u64 vcpu_get_supported_xss(struct kvm_vcpu *vcpu)
> > +{
> > +	struct kvm_cpuid_entry2 *best;
> > +
> > +	best = kvm_find_cpuid_entry_index(vcpu, 0xd, 1);
> > +	if (!best)
> > +		return 0;
> > +
> > +	return (best->ecx | ((u64)best->edx << 32)) & kvm_caps.supported_xss;
> > +}
> 
> Same question as one for patch that added vcpu_get_supported_xcr0()
> Why to have per vCPU supported XSS if we assume that all CPUs have the same
> CPUID?
> 
> I mean I am not against supporting hybrid CPU models, but KVM currently doesn't
> support this and this creates illusion that it does.

KVM does "support" hybrid vCPU models in the sense that KVM has allow hybrid models
since forever.  There are definite things that won't work, e.g. not all relevant
CPUID bits are captured in kvm_mmu_page_role, and so KVM will incorrectly share
page tables across vCPUs that are technically incompatible.

But for many features, heterogenous vCPU models do Just Work as far as KVM is
concerned.  There likely isn't a real world kernel that supports heterogenous
feature sets for things like XSS and XCR0, but that's a guest software limitation,
not a limitation of KVM's CPU virtualization.

As with many things, KVM's ABI is to let userspace shoot themselves in the foot.