linux-kernel - Re: [PATCH v6 04/16] KVM: x86/pmu: Set MSR_IA32_MISC_ENABLE

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Date:   Tue, 11 May 2021 18:58:39 -0700
From:   Venkatesh Srinivas <venkateshs@...omium.org>
To:     Like Xu <like.xu@...ux.intel.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Borislav Petkov <bp@...en8.de>,
        Sean Christopherson <seanjc@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, weijiang.yang@...el.com,
        Kan Liang <kan.liang@...ux.intel.com>, ak@...ux.intel.com,
        wei.w.wang@...el.com, eranian@...gle.com, liuxiangdong5@...wei.com,
        linux-kernel@...r.kernel.org, x86@...nel.org, kvm@...r.kernel.org,
        Yao Yuan <yuan.yao@...el.com>
Subject: Re: [PATCH v6 04/16] KVM: x86/pmu: Set MSR_IA32_MISC_ENABLE_EMON bit
 when vPMU is enabled

On 5/10/21, Like Xu <like.xu@...ux.intel.com> wrote:
> On Intel platforms, the software can use the IA32_MISC_ENABLE[7] bit to
> detect whether the processor supports performance monitoring facility.
>
> It depends on the PMU is enabled for the guest, and a software write
> operation to this available bit will be ignored.

Is the behavior that writes to IA32_MISC_ENABLE[7] are ignored (rather than #GP)
documented someplace?

Reviewed-by: Venkatesh Srinivas <venkateshs@...omium.org>

> Cc: Yao Yuan <yuan.yao@...el.com>
> Signed-off-by: Like Xu <like.xu@...ux.intel.com>
> ---
>  arch/x86/kvm/vmx/pmu_intel.c | 1 +
>  arch/x86/kvm/x86.c           | 1 +
>  2 files changed, 2 insertions(+)
>
> diff --git a/arch/x86/kvm/vmx/pmu_intel.c b/arch/x86/kvm/vmx/pmu_intel.c
> index 9efc1a6b8693..d9dbebe03cae 100644
> --- a/arch/x86/kvm/vmx/pmu_intel.c
> +++ b/arch/x86/kvm/vmx/pmu_intel.c
> @@ -488,6 +488,7 @@ static void intel_pmu_refresh(struct kvm_vcpu *vcpu)
>  	if (!pmu->version)
>  		return;
>
> +	vcpu->arch.ia32_misc_enable_msr |= MSR_IA32_MISC_ENABLE_EMON;
>  	perf_get_x86_pmu_capability(&x86_pmu);
>
>  	pmu->nr_arch_gp_counters = min_t(int, eax.split.num_counters,
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 5bd550eaf683..abe3ea69078c 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -3211,6 +3211,7 @@ int kvm_set_msr_common(struct kvm_vcpu *vcpu, struct
> msr_data *msr_info)
>  		}
>  		break;
>  	case MSR_IA32_MISC_ENABLE:
> +		data &= ~MSR_IA32_MISC_ENABLE_EMON;
>  		if (!kvm_check_has_quirk(vcpu->kvm, KVM_X86_QUIRK_MISC_ENABLE_NO_MWAIT)
> &&
>  		    ((vcpu->arch.ia32_misc_enable_msr ^ data) &
> MSR_IA32_MISC_ENABLE_MWAIT)) {
>  			if (!guest_cpuid_has(vcpu, X86_FEATURE_XMM3))
> --
> 2.31.1
>
>