linux-kernel - [PATCH 2/7] KVM: VMX: Give host userspace full control of MSR_IA32_PERF

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Date:   Sat, 11 Jun 2022 00:57:50 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     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>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, Like Xu <likexu@...cent.com>
Subject: [PATCH 2/7] KVM: VMX:  Give host userspace full control of MSR_IA32_PERF_CAPABILITIES

Do not clear manipulate MSR_IA32_PERF_CAPABILITIES in intel_pmu_refresh(),
i.e. give userspace full control over capability/read-only MSRs.  KVM is
not a babysitter, it is userspace's responsiblity to provide a valid and
coherent vCPU model.

Attempting to "help" the guest by forcing a consistent model creates edge
cases, and ironicially leads to inconsistent behavior.

Example #1:  KVM doesn't do intel_pmu_refresh() when userspace writes
the MSR.

Example #2: KVM doesn't clear the bits when the PMU is disabled, or when
there's no architectural PMU.

Signed-off-by: Sean Christopherson <seanjc@...gle.com>
---
 arch/x86/kvm/vmx/pmu_intel.c | 2 --
 1 file changed, 2 deletions(-)

diff --git a/arch/x86/kvm/vmx/pmu_intel.c b/arch/x86/kvm/vmx/pmu_intel.c
index 3b324ce0b142..b62012766226 100644
--- a/arch/x86/kvm/vmx/pmu_intel.c
+++ b/arch/x86/kvm/vmx/pmu_intel.c
@@ -619,8 +619,6 @@ static void intel_pmu_refresh(struct kvm_vcpu *vcpu)
 			pmu->pebs_enable_mask =
 				~((1ull << pmu->nr_arch_gp_counters) - 1);
 		}
-	} else {
-		vcpu->arch.perf_capabilities &= ~PERF_CAP_PEBS_MASK;
 	}
 }

-- 
2.36.1.476.g0c4daa206d-goog