linux-kernel - [PATCH v2 1/7] KVM: x86: Refresh PMU after writes to MSR_IA32_PERF

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Date:   Wed,  3 Aug 2022 19:26:52 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Sean Christopherson <seanjc@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>
Cc:     Mark Rutland <mark.rutland@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...nel.org>,
        Namhyung Kim <namhyung@...nel.org>,
        linux-perf-users@...r.kernel.org, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org, Like Xu <like.xu.linux@...il.com>
Subject: [PATCH v2 1/7] KVM: x86: Refresh PMU after writes to MSR_IA32_PERF_CAPABILITIES

Refresh the PMU if userspace modifies MSR_IA32_PERF_CAPABILITIES.  KVM
consumes the vCPU's PERF_CAPABILITIES when enumerating PEBS support, but
relies on CPUID updates to refresh the PMU.  I.e. KVM will do the wrong
thing if userspace stuffs PERF_CAPABILITIES _after_ setting guest CPUID.

Note, KVM may do the "wrong" thing if userspace changes PERF_CAPABILITIES
after running the vCPU, i.e. after KVM_RUN.  Similar to disallowing CPUID
changes after KVM_RUN, KVM should also disallow changing feature MSRs
after KVM_RUN to prevent unexpected behavior.  That problem will be
addressed separately at it affects MSRs other than PERF_CAPABILITES.

Opportunistically fix a curly-brace indentation.

Fixes: c59a1f106f5c ("KVM: x86/pmu: Add IA32_PEBS_ENABLE MSR emulation for extended PEBS")
Cc: Like Xu <like.xu.linux@...il.com>
Signed-off-by: Sean Christopherson <seanjc@...gle.com>
---
 arch/x86/kvm/x86.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 33560bfa0cac..dc19298e7150 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -3546,9 +3546,9 @@ int kvm_set_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
 			return 1;

 		vcpu->arch.perf_capabilities = data;
-
+		kvm_pmu_refresh(vcpu);
 		return 0;
-		}
+	}
 	case MSR_EFER:
 		return set_efer(vcpu, msr_info);
 	case MSR_K7_HWCR:
-- 
2.37.1.559.g78731f0fdb-goog