Re: [PATCH 02/15] KVM: x86: Emulate RDPID only if RDTSCP is supported

[Paolo Bonzini]

On 05/05/21 05:51, Reiji Watanabe wrote:
> > --- a/arch/x86/kvm/cpuid.c
> > +++ b/arch/x86/kvm/cpuid.c
> > @@ -637,7 +637,8 @@ static int __do_cpuid_func_emulated(struct kvm_cpuid_array *array, u32 func)
> >          case 7:
> >                  entry->flags |= KVM_CPUID_FLAG_SIGNIFCANT_INDEX;
> >                  entry->eax = 0;
> > -               entry->ecx = F(RDPID);
> > +               if (kvm_cpu_cap_has(X86_FEATURE_RDTSCP))
> > +                       entry->ecx = F(RDPID);
> >                  ++array->nent;
> >          default:
> >                  break;
> I'm wondering if entry->ecx should be set to F(RDPID) here
> even if the CPU supports RDPID natively.
> (i.e. kvm_cpu_cap_has(X86_FEATURE_RDPID) is true)
>
> The document "Documentation/virt/kvm/api.rst" says:
> ---
> 4.88 KVM_GET_EMULATED_CPUID
> ---------------------------
> <...>
> Userspace can use the information returned by this ioctl to query
> which features are emulated by kvm instead of being present natively.
> ---

Setting it always is consistent with the treatment of MOVBE above. 
Either way is okay but it should be done for both bits.

Paolo