linux-kernel - Re: [RESEND v13 09/10] KVM: vmx/pmu: Expose LBR_FMT in the MSR_IA32_PERF

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Date:   Wed, 27 Jan 2021 10:49:42 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     "Xu, Like" <like.xu@...el.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>,
        Wanpeng Li <wanpengli@...cent.com>,
        Like Xu <like.xu@...ux.intel.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>
Subject: Re: [RESEND v13 09/10] KVM: vmx/pmu: Expose LBR_FMT in the
 MSR_IA32_PERF_CAPABILITIES

On 27/01/21 06:45, Xu, Like wrote:
> On 2021/1/26 17:30, Paolo Bonzini wrote:
>> On 08/01/21 02:37, Like Xu wrote:
>>> Userspace could enable guest LBR feature when the exactly supported
>>> LBR format value is initialized to the MSR_IA32_PERF_CAPABILITIES
>>> and the LBR is also compatible with vPMU version and host cpu model.
>>>
>>> Signed-off-by: Like Xu <like.xu@...ux.intel.com>
>>> Reviewed-by: Andi Kleen <ak@...ux.intel.com>
>>> ---
>>>   arch/x86/kvm/vmx/capabilities.h | 9 ++++++++-
>>>   arch/x86/kvm/vmx/vmx.c          | 7 +++++++
>>>   2 files changed, 15 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/arch/x86/kvm/vmx/capabilities.h 
>>> b/arch/x86/kvm/vmx/capabilities.h
>>> index 57b940c613ab..a9a7c4d1b634 100644
>>> --- a/arch/x86/kvm/vmx/capabilities.h
>>> +++ b/arch/x86/kvm/vmx/capabilities.h
>>> @@ -378,7 +378,14 @@ static inline u64 vmx_get_perf_capabilities(void)
>>>        * Since counters are virtualized, KVM would support full
>>>        * width counting unconditionally, even if the host lacks it.
>>>        */
>>> -    return PMU_CAP_FW_WRITES;
>>> +    u64 perf_cap = PMU_CAP_FW_WRITES;
>>> +
>>> +    if (boot_cpu_has(X86_FEATURE_PDCM))
>>> +        rdmsrl(MSR_IA32_PERF_CAPABILITIES, perf_cap);
>>> +
>>> +    perf_cap |= perf_cap & PMU_CAP_LBR_FMT;
>>> +
>>> +    return perf_cap;
>>>   }
>>>     static inline u64 vmx_supported_debugctl(void)
>>> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
>>> index ad3b079f6700..9cb5b1e4fc27 100644
>>> --- a/arch/x86/kvm/vmx/vmx.c
>>> +++ b/arch/x86/kvm/vmx/vmx.c
>>> @@ -2229,6 +2229,13 @@ static int vmx_set_msr(struct kvm_vcpu *vcpu, 
>>> struct msr_data *msr_info)
>>>       case MSR_IA32_PERF_CAPABILITIES:
>>>           if (data && !vcpu_to_pmu(vcpu)->version)
>>>               return 1;
>>> +        if (data & PMU_CAP_LBR_FMT) {
>>> +            if ((data & PMU_CAP_LBR_FMT) !=
>>> +                (vmx_get_perf_capabilities() & PMU_CAP_LBR_FMT))
>>> +                return 1;
>>> +            if (!intel_pmu_lbr_is_compatible(vcpu))
>>> +                return 1;
>>> +        }
>>>           ret = kvm_set_msr_common(vcpu, msr_info);
>>>           break;
>>>
>>
>> Please move this hunk to patch 4.
>>
>> Paolo
>>
> Thanks, I'll do this part early in the next version.
> 
> I would have thought that we need to
> make the interface exposing as the last enabling step.

That's the right thing to do for vmx_get_perf_capabilities().  However, 
checking the values of MSR_IA32_PERF_CAPABILITIES can be done early.

Paolo