Re: [PATCH 3/9] KVM: x86: Initialize guest cpu_caps based on guest CPUID

[Yang, Weijiang]

On 11/17/2023 6:29 AM, Sean Christopherson wrote:
> On Thu, Nov 16, 2023, Weijiang Yang wrote:
> > On 11/11/2023 7:55 AM, Sean Christopherson wrote:
> >
> > [...]
> >
> > > -static __always_inline void guest_cpu_cap_check_and_set(struct kvm_vcpu *vcpu,
> > > -							unsigned int x86_feature)
> > > +static __always_inline void guest_cpu_cap_clear(struct kvm_vcpu *vcpu,
> > > +						unsigned int x86_feature)
> > >    {
> > > -	if (kvm_cpu_cap_has(x86_feature) && guest_cpuid_has(vcpu, x86_feature))
> > > +	unsigned int x86_leaf = __feature_leaf(x86_feature);
> > > +
> > > +	reverse_cpuid_check(x86_leaf);
> > > +	vcpu->arch.cpu_caps[x86_leaf] &= ~__feature_bit(x86_feature);
> > > +}
> > > +
> > > +static __always_inline void guest_cpu_cap_change(struct kvm_vcpu *vcpu,
> > > +						 unsigned int x86_feature,
> > > +						 bool guest_has_cap)
> > > +{
> > > +	if (guest_has_cap)
> > >    		guest_cpu_cap_set(vcpu, x86_feature);
> > > +	else
> > > +		guest_cpu_cap_clear(vcpu, x86_feature);
> > > +}
> > I don't see any necessity to add 3 functions, i.e., guest_cpu_cap_{set, clear, change}, for
> I want to have equivalents to the cpuid_entry_*() APIs so that we don't end up
> with two different sets of names.  And the clear() API already has a second user.
>
> > guest_cpu_cap update. IMHO one function is enough, e.g,:
> Hrm, I open coded the OR/AND logic in cpuid_entry_change() to try to force CMOV
> instead of Jcc.  That honestly seems like a pointless optimization.  I would
> rather use the helpers, which is less code.
>
> > static __always_inline void guest_cpu_cap_update(struct kvm_vcpu *vcpu,
> >                                                   unsigned int x86_feature,
> >                                                   bool guest_has_cap)
> > {
> >          unsigned int x86_leaf = __feature_leaf(x86_feature);
> >
> > reverse_cpuid_check(x86_leaf);
> >          if (guest_has_cap)
> >                  vcpu->arch.cpu_caps[x86_leaf] |= __feature_bit(x86_feature);
> > else
> >                  vcpu->arch.cpu_caps[x86_leaf] &= ~__feature_bit(x86_feature);
> > }
> >
> > > +
> > > +static __always_inline void guest_cpu_cap_restrict(struct kvm_vcpu *vcpu,
> > > +						   unsigned int x86_feature)
> > > +{
> > > +	if (!kvm_cpu_cap_has(x86_feature))
> > > +		guest_cpu_cap_clear(vcpu, x86_feature);
> > >    }
> > _restrict is not clear to me for what the function actually does -- it
> > conditionally clears guest cap depending on KVM support of the feature.
> >
> > How about renaming it to guest_cpu_cap_sync()?
> "sync" isn't correct because it's not synchronizing with KVM's capabilitiy, e.g.
> the guest capability will remaing unset if the guest CPUID bit is clear but the
> KVM capability is available.
>
> How about constrain()?
I don't know, just feel we already have guest_cpu_cap_{set, clear, change}, here the name cannot exactly match the behavior of the function, maybe guest_cpu_cap_filter()? But just ignore the nit, up to you to decide the name :-)