linux-kernel - Re: [PATCH v2 7/7] kvm: x86/cpuid: Fix CPUID leaf 0xA

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Date:   Tue, 26 Apr 2022 14:10:43 +0530
From:   Sandipan Das <sandipan.das@....com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     linux-kernel@...r.kernel.org, linux-perf-users@...r.kernel.org,
        x86@...nel.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, like.xu.linux@...il.com,
        eranian@...gle.com, ananth.narayan@....com, ravi.bangoria@....com,
        santosh.shukla@....com, puwen@...on.cn
Subject: Re: [PATCH v2 7/7] kvm: x86/cpuid: Fix CPUID leaf 0xA

Hi Peter,

On 4/26/2022 1:45 PM, Peter Zijlstra wrote:
> On Thu, Apr 21, 2022 at 11:16:59AM +0530, 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
>> 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@....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..6bd65cad75ef 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 (boot_cpu_data.x86_vendor == X86_VENDOR_AMD) {
> 
> This is the obligatory question about HYGON; should they be included
> here?
> 
> x86 is getting a number of me-too patches from both sides, where
> behaviour has diverged for no raisin and then needs to be fixed up
> again.
> 

>From what I understand, Hygon processors also do not support this leaf.
Copying Pu Wen for confirmation.

- Sandipan

>> +			entry->eax = entry->ebx = entry->ecx = entry->edx = 0;
>> +			break;
>> +		}
>> +
>>  		perf_get_x86_pmu_capability(&cap);
>>  
>>  		/*
>> -- 
>> 2.32.0
>>