Re: [PATCH v3 5/8] KVM: x86/cpuid: Add X86_FEATURE_AMD_PMU_V2 as a KVM-only leaf entry

[Like Xu]

On 25/1/2023 3:47 am, Sean Christopherson wrote:
> On Fri, Nov 11, 2022, Like Xu wrote:
> > From: Like Xu <likexu@xxxxxxxxxxx>
> >
> > Alias X86_FEATURE_AMD_PMU_V2 for feature AMD_PMU_V2 in KVM-only leafs that
> > aren't scattered by cpufeatures.h so that it can be used in KVM, e.g. to
> > query guest CPUID.  As a bonus, no translation is needed for these features
> > in __feature_translate().
> >
> > Suggested-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> > Signed-off-by: Like Xu <likexu@xxxxxxxxxxx>
> > ---
> >   arch/x86/kvm/reverse_cpuid.h | 10 ++++++++++
> >   1 file changed, 10 insertions(+)
> >
> > diff --git a/arch/x86/kvm/reverse_cpuid.h b/arch/x86/kvm/reverse_cpuid.h
> > index a19d473d0184..7cfedb3e47c0 100644
> > --- a/arch/x86/kvm/reverse_cpuid.h
> > +++ b/arch/x86/kvm/reverse_cpuid.h
> > @@ -13,6 +13,7 @@
> >    */
> >   enum kvm_only_cpuid_leafs {
> >   	CPUID_12_EAX	 = NCAPINTS,
> > +	CPUID_8000_0022_EAX,
> >   	NR_KVM_CPU_CAPS,
> >   
> >   	NKVMCAPINTS = NR_KVM_CPU_CAPS - NCAPINTS,
> > @@ -23,7 +24,15 @@ enum kvm_only_cpuid_leafs {
> >   /* Intel-defined SGX sub-features, CPUID level 0x12 (EAX). */
> >   #define KVM_X86_FEATURE_SGX1		KVM_X86_FEATURE(CPUID_12_EAX, 0)
> >   #define KVM_X86_FEATURE_SGX2		KVM_X86_FEATURE(CPUID_12_EAX, 1)
> > +#define KVM_X86_FEATURE_AMD_PMU_V2	KVM_X86_FEATURE(CPUID_8000_0022_EAX, 0)
> >   
> > +/*
> > + * Alias X86_FEATURE_* to the KVM variant for features in KVM-only leafs that
> > + * aren't scattered by cpufeatures.h so that X86_FEATURE_* can be used in KVM,
> > + * e.g. to query guest CPUID.  As a bonus, no translation is needed for these
> > + * features in __feature_translate().
> > + */
> > +#define X86_FEATURE_AMD_PMU_V2      KVM_X86_FEATURE_AMD_PMU_V2
>
> I gave you bad input earlier, for purely KVM-defined flags there's no need for an
> intermediate KVM_X86_FEATURE_AMD_PMU_V2, this could simply be:
>
>    #define X86_FEATURE_AMD_PMU_V2         KVM_X86_FEATURE(CPUID_8000_0022_EAX, 0)
>
> That's a moot point though because, after much searching because I had a very hard
> time believing the kernel wouldn't want to know about this flag, I found commit
>
>    d6d0c7f681fd ("x86/cpufeatures: Add PerfMonV2 feature bit")
>
> from nearly a year ago.  I.e. to avoid confusiong, this needs to be a scattered
> flag, not a purely KVM flag.
>
> ---
>   arch/x86/kvm/reverse_cpuid.h | 7 +++++++
>   1 file changed, 7 insertions(+)
>
> diff --git a/arch/x86/kvm/reverse_cpuid.h b/arch/x86/kvm/reverse_cpuid.h
> index 4945456fd646..333e28b0a13c 100644
> --- a/arch/x86/kvm/reverse_cpuid.h
> +++ b/arch/x86/kvm/reverse_cpuid.h
> @@ -15,6 +15,7 @@ enum kvm_only_cpuid_leafs {
>   	CPUID_12_EAX	 = NCAPINTS,
>   	CPUID_7_1_EDX,
>   	CPUID_8000_0007_EDX,
> +	CPUID_8000_0022_EAX,
>   	NR_KVM_CPU_CAPS,
>   
>   	NKVMCAPINTS = NR_KVM_CPU_CAPS - NCAPINTS,
> @@ -47,6 +48,9 @@ enum kvm_only_cpuid_leafs {
>   /* CPUID level 0x80000007 (EDX). */
>   #define KVM_X86_FEATURE_CONSTANT_TSC	KVM_X86_FEATURE(CPUID_8000_0007_EDX, 8)
>   
> +/* CPUID level 0x80000022 (EAX) */
> +#define KVM_X86_FEATURE_PERFMON_V2	KVM_X86_FEATURE(CPUID_8000_0022_EAX, 0)

I'm very confused and is this the usage you want to see:

	kvm_cpu_cap_set(KVM_X86_FEATURE_PERFMON_V2)
	kvm_cpu_cap_has(KVM_X86_FEATURE_PERFMON_V2)
	guest_cpuid_has(vcpu, X86_FEATURE_PERFMON_V2)

? then what about X86_FEATURE_PERFMON_V2 ?

> +
>   struct cpuid_reg {
>   	u32 function;
>   	u32 index;
> @@ -73,6 +77,7 @@ static const struct cpuid_reg reverse_cpuid[] = {
>   	[CPUID_8000_001F_EAX] = {0x8000001f, 0, CPUID_EAX},
>   	[CPUID_7_1_EDX]       = {         7, 1, CPUID_EDX},
>   	[CPUID_8000_0007_EDX] = {0x80000007, 0, CPUID_EDX},
> +	[CPUID_8000_0022_EAX] = {0x80000022, 0, CPUID_EAX},
>   };
>   
>   /*
> @@ -107,6 +112,8 @@ static __always_inline u32 __feature_translate(int x86_feature)
>   		return KVM_X86_FEATURE_SGX_EDECCSSA;
>   	else if (x86_feature == X86_FEATURE_CONSTANT_TSC)
>   		return KVM_X86_FEATURE_CONSTANT_TSC;
> +	else if (x86_feature == X86_FEATURE_PERFMON_V2)
> +		return KVM_X86_FEATURE_PERFMON_V2;
>   
>   	return x86_feature;
>   }
>
> base-commit: 5f3f3cc1279cd5cd52d301b97844bd3ce40c8020