linux-kernel - Re: [PATCH 3/3] KVM: VMX: Adjust number of LBR records for PERF

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Message-ID: <2d932ad7-899b-ed26-d77c-f149fb2afc36@gmail.com>
Date:   Fri, 29 Jul 2022 18:10:11 +0800
From:   Like Xu <like.xu.linux@...il.com>
To:     Sean Christopherson <seanjc@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/3] KVM: VMX: Adjust number of LBR records for
 PERF_CAPABILITIES at refresh

On 28/7/2022 7:34 am, Sean Christopherson wrote:
> guest_cpuid_has() is expensive due to the linear search of guest CPUID
> entries, intel_pmu_lbr_is_enabled() is checked on every VM-Enter,_and_
> simply enumerating the same "Model" as the host causes KVM to set the
> number of LBR records to a non-zero value.

Before reconsidering vcpu->arch.perf_capabilities to reach a conclusion,
how about this minor inline change help reduce my sins ?

diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index 0ecbbae42976..06a21d66be13 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -7039,7 +7039,8 @@ static fastpath_t vmx_vcpu_run(struct kvm_vcpu *vcpu)
  	pt_guest_enter(vmx);

  	atomic_switch_perf_msrs(vmx);
-	if (intel_pmu_lbr_is_enabled(vcpu))
+	if (vmx->lbr_desc.records.nr &&
+	    (vcpu->arch.perf_capabilities & PMU_CAP_LBR_FMT))
  		vmx_passthrough_lbr_msrs(vcpu);

  	if (enable_preemption_timer)