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

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Date:   Wed, 27 Jan 2021 10:48:50 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Like Xu <like.xu@...ux.intel.com>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Borislav Petkov <bp@...en8.de>,
        "H . Peter Anvin" <hpa@...or.com>, ak@...ux.intel.com,
        wei.w.wang@...el.com, kan.liang@...el.com, x86@...nel.org,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        Joerg Roedel <joro@...tes.org>,
        Jim Mattson <jmattson@...gle.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>
Subject: Re: [RESEND v13 03/10] KVM: x86/pmu: Use IA32_PERF_CAPABILITIES to
 adjust features visibility

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