[PATCH 2/7] KVM: VMX: Give host userspace full control of MSR_IA32_PERF_CAPABILITIES

Sean Christopherson
2022-06-11T00:58:14Z

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>
---
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