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Date:   Tue, 3 May 2022 14:04:04 +0200
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Subject: Re: [PATCH v4 7/7] kvm: x86/cpuid: Fix CPUID leaf 0xA
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To:     Jim Mattson <jmattson@google.com>,
        Sandipan Das <sandipan.das@amd.com>
Cc:     linux-kernel@vger.kernel.org, linux-perf-users@vger.kernel.org,
        x86@kernel.org, peterz@infradead.org, bp@alien8.de,
        dave.hansen@linux.intel.com, acme@kernel.org, mark.rutland@arm.com,
        alexander.shishkin@linux.intel.com, namhyung@kernel.org,
        jolsa@kernel.org, tglx@linutronix.de, mingo@redhat.com,
        like.xu.linux@gmail.com, eranian@google.com, puwen@hygon.cn,
        ananth.narayan@amd.com, ravi.bangoria@amd.com,
        santosh.shukla@amd.com
References: <cover.1651058600.git.sandipan.das@amd.com>
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 <CALMp9eSdUW6et2Us1if-j2Do6cv9ssyT4C2KPiyomFd7i_Dc1Q@mail.gmail.com>
From:   Paolo Bonzini <pbonzini@redhat.com>
In-Reply-To: <CALMp9eSdUW6et2Us1if-j2Do6cv9ssyT4C2KPiyomFd7i_Dc1Q@mail.gmail.com>
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On 5/3/22 01:50, Jim Mattson wrote:
> On Wed, Apr 27, 2022 at 4:34 AM Sandipan Das <sandipan.das@amd.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@amd.com>
>> Signed-off-by: Sandipan Das <sandipan.das@amd.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?

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