Re: [PATCH][v6] KVM: X86: support APERF/MPERF registers

[Jim Mattson]

On Thu, Jun 4, 2020 at 11:35 PM Paolo Bonzini <pbonzini@xxxxxxxxxx> wrote:
>
> On 05/06/20 07:00, Xiaoyao Li wrote:
> > you could do
> >
> >     bool guest_cpuid_aperfmperf = false;
> >     if (best)
> >         guest_cpuid_aperfmperf = !!(best->ecx & BIT(0));
> >
> >     if (guest_cpuid_aperfmerf != guest_has_aperfmperf(vcpu->kvm))
> >         return -EINVAL;
> >
> >
> > In fact, I think we can do nothing here. Leave it as what usersapce
> > wants just like how KVM treats other CPUID bits.
>
> The reason to do it like Rongqing did is that it's suggested to take the
> output of KVM_GET_SUPPORTED_CPUID and pass it down to KVM_SET_CPUID2.
> Unfortunately we have KVM_GET_SUPPORTED_CPUID as a /dev/kvm (not VM)
> ioctl, otherwise you could have used guest_has_aperfmperf to affect the
> output of KVM_GET_SUPPORTED_CPUID.
>
> I think it's okay however to keep it simple as you suggest.  In this
> case however __do_cpuid_func must not return the X86_FEATURE_APERFMPERF bit.
>
> The guest can instead check for the availability of KVM_CAP_APERFMPERF,
> which is already done in Rongqing's patch.
>
> >> @@ -4930,6 +4939,11 @@ int kvm_vm_ioctl_enable_cap(struct kvm *kvm,
> >>           kvm->arch.exception_payload_enabled = cap->args[0];
> >>           r = 0;
> >>           break;
> >> +    case KVM_CAP_APERFMPERF:
> >> +        kvm->arch.aperfmperf_mode =
> >> +            boot_cpu_has(X86_FEATURE_APERFMPERF) ? cap->args[0] : 0;
> >
> > Shouldn't check whether cap->args[0] is a valid value?
>
> Yes, only valid values should be allowed.
>
> Also, it should fail with -EINVAL if the host does not have
> X86_FEATURE_APERFMPERF.

Should enabling/disabling this capability be disallowed once vCPUs
have been created?