Re: [PATCH v2 44/49] KVM: x86: Update guest cpu_caps at runtime for dynamic CPUID-based features

[Maxim Levitsky]

On Mon, 2024-07-08 at 17:24 -0700, Sean Christopherson wrote:
> On Thu, Jul 04, 2024, Maxim Levitsky wrote:
> > On Fri, 2024-05-17 at 10:39 -0700, Sean Christopherson wrote:
> > > -		cpuid_entry_change(best, X86_FEATURE_OSPKE,
> > > -				   kvm_is_cr4_bit_set(vcpu, X86_CR4_PKE));
> > > +		kvm_update_feature_runtime(vcpu, best, X86_FEATURE_OSPKE,
> > > +					   kvm_is_cr4_bit_set(vcpu, X86_CR4_PKE));
> > > +
> > >  
> > >  	best = kvm_find_cpuid_entry_index(vcpu, 0xD, 0);
> > >  	if (best)
> > 
> > I am not 100% sure that we need to do this.
> > 
> > Runtime cpuid changes are a hack that Intel did back then, due to various
> > reasons, These changes don't really change the feature set that CPU supports,
> > but merly as you like to say 'massage' the output of the CPUID instruction to
> > make the unmodified OS happy usually.
> > 
> > Thus it feels to me that CPU caps should not include the dynamic features,
> > and neither KVM should use the value of these as a source for truth, but
> > rather the underlying source of the truth (e.g CR4).
> > 
> > But if you insist, I don't really have a very strong reason to object this.
> 
> FWIW, I think I agree that CR4 should be the source of truth, but it's largely a
> moot point because KVM doesn't actually check OSXSAVE or OSPKE, as KVM never
> emulates the relevant instructions.  So for those, it's indeed not strictly
> necessary.
> 
> Unfortunately, KVM has established ABI for checking X86_FEATURE_MWAIT when
> "emulating" MONITOR and MWAIT, i.e. KVM can't use vcpu->arch.ia32_misc_enable_msr
> as the source of truth.

Can you elaborate on this? Can you give me an example of the ABI?


>   So for MWAIT, KVM does need to update CPU caps (or carry
> even more awful MWAIT code), at which point extending the behavior to the CR4
> features (and to X86_FEATURE_APIC) is practically free.
> 


Best regards,
	Maxim Levitsky