linux-kernel - [PATCH] KVM: x86/pmu: Disallow legacy LBRs if architectural LBRs are available

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Message-ID: <20230128001427.2548858-1-seanjc@google.com>
Date:   Sat, 28 Jan 2023 00:14:27 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Sean Christopherson <seanjc@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        Yang Weijiang <weijiang.yang@...el.com>,
        Like Xu <like.xu.linux@...il.com>
Subject: [PATCH] KVM: x86/pmu: Disallow legacy LBRs if architectural LBRs are available

Disallow enabling LBR support if the CPU supports architectural LBRs.
Traditional LBR support is absent on CPU models that have architectural
LBRs, and KVM doesn't yet support arch LBRs, i.e. KVM will pass through
non-existent MSRs if userspace enables LBRs for the guest.

Cc: stable@...r.kernel.org
Cc: Yang Weijiang <weijiang.yang@...el.com>
Cc: Like Xu <like.xu.linux@...il.com>
Reported-by: Paolo Bonzini <pbonzini@...hat.com>
Signed-off-by: Sean Christopherson <seanjc@...gle.com>
---

Am I missing something that would prevent this scenario?

 arch/x86/kvm/vmx/vmx.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index 8f0f67c75f35..77ee6b4a5ec4 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -7761,9 +7761,11 @@ static u64 vmx_get_perf_capabilities(void)
 	if (boot_cpu_has(X86_FEATURE_PDCM))
 		rdmsrl(MSR_IA32_PERF_CAPABILITIES, host_perf_cap);
 
-	x86_perf_get_lbr(&lbr);
-	if (lbr.nr)
-		perf_cap |= host_perf_cap & PMU_CAP_LBR_FMT;
+	if (!cpu_feature_enabled(X86_FEATURE_ARCH_LBR)) {
+		x86_perf_get_lbr(&lbr);
+		if (lbr.nr)
+			perf_cap |= host_perf_cap & PMU_CAP_LBR_FMT;
+	}
 
 	if (vmx_pebs_supported()) {
 		perf_cap |= host_perf_cap & PERF_CAP_PEBS_MASK;

base-commit: 2de154f541fc5b9f2aed3fe06e218130718ce320
-- 
2.39.1.456.gfc5497dd1b-goog