Re: [RESEND v13 03/10] KVM: x86/pmu: Use IA32_PERF_CAPABILITIES to adjust features visibility

Paolo Bonzini
2021-01-27T09:54:09Z

On 27/01/21 07:04, Like Xu wrote:
> On 2021/1/26 17:42, Paolo Bonzini wrote:
>> On 08/01/21 02:36, Like Xu wrote:
>>>
>>> @@ -401,6 +398,9 @@ static void intel_pmu_init(struct kvm_vcpu *vcpu)
>>>          pmu->fixed_counters[i].idx = i + INTEL_PMC_IDX_FIXED;
>>>          pmu->fixed_counters[i].current_config = 0;
>>>      }
>>> +
>>> +    vcpu->arch.perf_capabilities = guest_cpuid_has(vcpu,
>>> X86_FEATURE_PDCM) ?
>>> +        vmx_get_perf_capabilities() : 0;
>>
>> There is one thing I don't understand with this patch: intel_pmu_init
>> is not called when CPUID is changed.  So I would have thought that
>> anything that uses guest_cpuid_has must stay in intel_pmu_refresh.  As
>> I understand it vcpu->arch.perf_capabilities is always set to 0
>> (vmx_get_perf_capabilities is never called), and kvm_set_msr_common
>> would fail to set any bit in the MSR.  What am I missing?
>>
>> In addition, the code of patch 4:
>>
>> +    if (!intel_pmu_lbr_is_enabled(vcpu)) {
>> +        vcpu->arch.perf_capabilities &= ~PMU_CAP_LBR_FMT;
>> +        lbr_desc->records.nr = 0;
>> +    }
>>
>> is not okay after MSR changes.  The value written by the host must be
>> either rejected (with "return 1") or applied unchanged.
>>
>> Fortunately I think this code is dead if you move the check in
>> kvm_set_msr from patch 9 to patch 4.  However, in patch 9
>> vmx_get_perf_capabilities() must only set the LBR format bits if
>> intel_pmu_lbr_is_compatible(vcpu).
>
> Thanks for the guidance. How about handling it in this way:
>
> In the intel_pmu_init():
>
>     vcpu->arch.perf_capabilities = 0;
>     lbr_desc->records.nr = 0;
>
> In the intel_pmu_refresh():
>
>     if (guest_cpuid_has(vcpu, X86_FEATURE_PDCM)) {
>         vcpu->arch.perf_capabilities = vmx_get_perf_capabilities();
>         if (!lbr_desc->records.nr)
>             vcpu->arch.perf_capabilities &= ~PMU_CAP_LBR_FMT;
>     }
>
> In the vmx_set_msr():
>
>     case MSR_IA32_PERF_CAPABILITIES:
>         // set up lbr_desc->records.nr
>         if (!intel_pmu_lbr_is_compatible(vcpu))

Maybe pass msr_info.data to intel_pmu_lbr_is_compatible? Otherwise
seems okay, thanks Like.

Paolo