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

[Sean Christopherson]

On Mon, Feb 06, 2023, Like Xu wrote:
> 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)
> 
> ?

No, all of those should just use X86_FEATURE_PERFMON_V2, i.e. the existing flag
from cpufeatures.h.

> then what about X86_FEATURE_PERFMON_V2 ?

Not sure I follow.  As above, it's scattered, thus KVM needs to redirect it to
the hardware-defined bit position, which is the role of __feature_translate()
and KVM_X86_FEATURE_PERFMON_V2.