linux-kernel - Re: [PATCH v3 7/7] kvm: x86/cpuid: Fix CPUID leaf 0xA

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Date:   Wed, 27 Apr 2022 15:22:02 +0530
From:   Sandipan Das <sandipan.das@....com>
To:     Like Xu <like.xu.linux@...il.com>
Cc:     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, jmattson@...gle.com, eranian@...gle.com,
        puwen@...on.cn, ananth.narayan@....com, ravi.bangoria@....com,
        santosh.shukla@....com, x86@...nel.org,
        linux-perf-users@...r.kernel.org,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Vasant Hegde <vasant.hegde@....com>
Subject: Re: [PATCH v3 7/7] kvm: x86/cpuid: Fix CPUID leaf 0xA


On 4/27/2022 3:02 PM, Like Xu wrote:
> On 26/4/2022 9:05 pm, Sandipan Das 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
> 
> So, why not:
> 
>         if (!static_cpu_has(X86_FEATURE_ARCH_PERFMON))
>             break;
> ?
> 

That seems to be a better option. Thanks for the suggestion!
Will change it in the next revision.

>> 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 | 6 ++++++
>>   1 file changed, 6 insertions(+)
>>
>> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
>> index 4b62d80bb22f..d27d6a8f601a 100644
>> --- a/arch/x86/kvm/cpuid.c
>> +++ b/arch/x86/kvm/cpuid.c
>> @@ -872,6 +872,12 @@ 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 ||
>> +            boot_cpu_data.x86_vendor == X86_VENDOR_HYGON) {
>> +            entry->eax = entry->ebx = entry->ecx = entry->edx = 0;
>> +            break;
>> +        }
>> +
>>           perf_get_x86_pmu_capability(&cap);
>>             /*