Re: [PATCH 7/7] kvm: x86/cpuid: Fix Architectural Performance Monitoring support

[Peter Zijlstra]

On Thu, Mar 17, 2022 at 11:58:36AM +0530, Sandipan Das wrote:
> CPUID 0xA provides information on Architectural Performance
> Monitoring features on some x86 processors. 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 CPUID function and additional MSRs are not supported on
> AMD processors. If 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 b8f8d268d058..1d9ca5726167 100644
> --- a/arch/x86/kvm/cpuid.c
> +++ b/arch/x86/kvm/cpuid.c
> @@ -865,6 +865,11 @@ static inline int __do_cpuid_func(struct kvm_cpuid_array *array, u32 function)
>  		union cpuid10_eax eax;
>  		union cpuid10_edx edx;
>  
> +		if (boot_cpu_data.x86_vendor == X86_VENDOR_AMD) {
> +			entry->eax = entry->ebx = entry->ecx = entry->edx = 0;
> +			break;
> +		}
> +

Because actually implementing perfmon-v2 would've been too convenient,
right? IIRC you're very close to actually supporing perfmon-v2
capability wise here.