Re: [PATCH 6/9] KVM: x86: Update guest cpu_caps at runtime for dynamic CPUID-based features

[Sean Christopherson]

On Mon, Nov 13, 2023, Robert Hoo wrote:
> On 11/11/2023 7:55 AM, Sean Christopherson wrote:
> > When updating guest CPUID entries to emulate runtime behavior, e.g. when
> > the guest enables a CR4-based feature that is tied to a CPUID flag, also
> > update the vCPU's cpu_caps accordingly.  This will allow replacing all
> > usage of guest_cpuid_has() with guest_cpu_cap_has().
> > 
> > Take care not to update guest capabilities when KVM is updating CPUID
> > entries that *may* become the vCPU's CPUID, e.g. if userspace tries to set
> > bogus CPUID information.  No extra call to update cpu_caps is needed as
> > the cpu_caps are initialized from the incoming guest CPUID, i.e. will
> > automatically get the updated values.
> > 
> > Note, none of the features in question use guest_cpu_cap_has() at this
> > time, i.e. aside from settings bits in cpu_caps, this is a glorified nop.
> > 
> > Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> > ---
> >   arch/x86/kvm/cpuid.c | 48 +++++++++++++++++++++++++++++++-------------
> >   1 file changed, 34 insertions(+), 14 deletions(-)
> > 
> > diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> > index 36bd04030989..37a991439fe6 100644
> > --- a/arch/x86/kvm/cpuid.c
> > +++ b/arch/x86/kvm/cpuid.c
> > @@ -262,31 +262,48 @@ static u64 cpuid_get_supported_xcr0(struct kvm_cpuid_entry2 *entries, int nent)
> >   	return (best->eax | ((u64)best->edx << 32)) & kvm_caps.supported_xcr0;
> >   }
> > +static __always_inline void kvm_update_feature_runtime(struct kvm_vcpu *vcpu,
> > +						       struct kvm_cpuid_entry2 *entry,
> > +						       unsigned int x86_feature,
> > +						       bool has_feature)
> > +{
> > +	if (entry)
> > +		cpuid_entry_change(entry, x86_feature, has_feature);
> > +
> > +	if (vcpu)
> > +		guest_cpu_cap_change(vcpu, x86_feature, has_feature);
> > +}
> > +
> >   static void __kvm_update_cpuid_runtime(struct kvm_vcpu *vcpu, struct kvm_cpuid_entry2 *entries,
> >   				       int nent)
> >   {
> >   	struct kvm_cpuid_entry2 *best;
> > +	struct kvm_vcpu *caps = vcpu;
> 
> u32 *caps  = vcpu->arch.cpu_caps;
> and update guest_cpu_cap_set(), guest_cpu_cap_clear(),
> guest_cpu_cap_change() and guest_cpu_cap_restrict() to pass in
> vcpu->arch.cpu_caps instead of vcpu, since all of them merely refer to vcpu
> cap, rather than whole vcpu info.

No, because then every caller would need extra code to pass vcpu->cpu_caps, and
passing 'u32 *' provides less type safety than 'struct kvm_vcpu *'.  That tradeoff
isn't worth making this one path slightly easier to read.

> Or, for simple change, here rename variable name "caps" --> "vcpu", to less
> reading confusion.

@vcpu is already defined and needs to be used in this function.  See the comment
below.

I'm definitely open to a better name, though I would like to keep the name
relative short so that the line lengths of the callers is reasonable, e.g. would
prefer not to do vcpu_caps.

> > +	/*
> > +	 * Don't update vCPU capabilities if KVM is updating CPUID entries that
> > +	 * are coming in from userspace!
> > +	 */
> > +	if (entries != vcpu->arch.cpuid_entries)
> > +		caps = NULL;
> >   	best = cpuid_entry2_find(entries, nent, 1, KVM_CPUID_INDEX_NOT_SIGNIFICANT);
> > -	if (best) {
> > -		/* Update OSXSAVE bit */
> > -		if (boot_cpu_has(X86_FEATURE_XSAVE))
> > -			cpuid_entry_change(best, X86_FEATURE_OSXSAVE,
> > +
> > +	if (boot_cpu_has(X86_FEATURE_XSAVE))
> > +		kvm_update_feature_runtime(caps, best, X86_FEATURE_OSXSAVE,
> >   					   kvm_is_cr4_bit_set(vcpu, X86_CR4_OSXSAVE));
> > -		cpuid_entry_change(best, X86_FEATURE_APIC,
> > -			   vcpu->arch.apic_base & MSR_IA32_APICBASE_ENABLE);
> > +	kvm_update_feature_runtime(caps, best, X86_FEATURE_APIC,
> > +				   vcpu->arch.apic_base & MSR_IA32_APICBASE_ENABLE);