Re: [PATCH v4 7/7] kvm: x86/cpuid: Fix CPUID leaf 0xA

[Paolo Bonzini]

On 5/3/22 01:50, Jim Mattson wrote:
> On Wed, Apr 27, 2022 at 4:34 AM Sandipan Das <sandipan.das@xxxxxxx> wrote:
> > On some x86 processors, CPUID leaf 0xA provides information
> > on Architectural Performance Monitoring features. It
> > advertises a PMU version which Qemu uses to determine the
> > availability of additional MSRs to manage the PMCs.
> >
> > Upon receiving a KVM_GET_SUPPORTED_CPUID ioctl request for
> > the same, the kernel constructs return values based on the
> > x86_pmu_capability irrespective of the vendor.
> >
> > This leaf and the additional MSRs are not supported on AMD
> > and Hygon processors. If AMD PerfMonV2 is detected, the PMU
> > version is set to 2 and guest startup breaks because of an
> > attempt to access a non-existent MSR. Return zeros to avoid
> > this.
> >
> > Fixes: a6c06ed1a60a ("KVM: Expose the architectural performance monitoring CPUID leaf")
> > Reported-by: Vasant Hegde <vasant.hegde@xxxxxxx>
> > Signed-off-by: Sandipan Das <sandipan.das@xxxxxxx>
> > ---
> >   arch/x86/kvm/cpuid.c | 5 +++++
> >   1 file changed, 5 insertions(+)
> >
> > diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> > index 4b62d80bb22f..e66ebb747084 100644
> > --- a/arch/x86/kvm/cpuid.c
> > +++ b/arch/x86/kvm/cpuid.c
> > @@ -872,6 +872,11 @@ static inline int __do_cpuid_func(struct kvm_cpuid_array *array, u32 function)
> >                  union cpuid10_eax eax;
> >                  union cpuid10_edx edx;
> >
> > +               if (!static_cpu_has(X86_FEATURE_ARCH_PERFMON)) {
>
> Should this be checking kvm_cpu_cap_has(X86_FEATURE_ARCH_PERFMON) instead?

Ah, it cannot because X86_FEATURE_ARCH_PERFMON is a synthetic feature. 
kvm_cpu_cap_has only works with features that are backed by CPUID bits.

Paolo