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KVM: VMX: Give host userspace full control of MSR_IA32_PERF_CAPABILITIES

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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@google.com>
Message-Id: <20220611005755.753273-3-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
This commit is contained in:
Sean Christopherson 2022-06-11 00:57:50 +00:00 committed by Paolo Bonzini
parent 9fc222967a
commit 0f4a718527
1 changed files with 0 additions and 2 deletions
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2
arch/x86/kvm/vmx/pmu_intel.c
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@ -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;
}
}