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

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Date:   Mon, 2 May 2022 16:50:32 -0700
From:   Jim Mattson <jmattson@...gle.com>
To:     Sandipan Das <sandipan.das@....com>
Cc:     linux-kernel@...r.kernel.org, linux-perf-users@...r.kernel.org,
        x86@...nel.org, peterz@...radead.org, bp@...en8.de,
        dave.hansen@...ux.intel.com, acme@...nel.org, mark.rutland@....com,
        alexander.shishkin@...ux.intel.com, namhyung@...nel.org,
        jolsa@...nel.org, tglx@...utronix.de, mingo@...hat.com,
        pbonzini@...hat.com, like.xu.linux@...il.com, eranian@...gle.com,
        puwen@...on.cn, ananth.narayan@....com, ravi.bangoria@....com,
        santosh.shukla@....com
Subject: Re: [PATCH v4 7/7] kvm: x86/cpuid: Fix CPUID leaf 0xA

On Wed, Apr 27, 2022 at 4:34 AM Sandipan Das <sandipan.das@....com> 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@....com>
> Signed-off-by: Sandipan Das <sandipan.das@....com>
> ---
>  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?

> +                       entry->eax = entry->ebx = entry->ecx = entry->edx = 0;
> +                       break;
> +               }
> +
>                 perf_get_x86_pmu_capability(&cap);
>
>                 /*
> --
> 2.34.1
>