[PATCH 3/3] KVM: VMX: Disable LBR virtualization if the CPU doesn't support LBR callstacks

From: Sean Christopherson <seanjc@google.com>
To: Peter Zijlstra <peterz@infradead.org>,
	Ingo Molnar <mingo@redhat.com>,
	 Arnaldo Carvalho de Melo <acme@kernel.org>,
	Namhyung Kim <namhyung@kernel.org>,
	 Sean Christopherson <seanjc@google.com>,
	Paolo Bonzini <pbonzini@redhat.com>
Cc: linux-perf-users@vger.kernel.org, linux-kernel@vger.kernel.org,
	 kvm@vger.kernel.org, Mingwei Zhang <mizhang@google.com>,
	Jim Mattson <jmattson@google.com>
Subject: [PATCH 3/3] KVM: VMX: Disable LBR virtualization if the CPU doesn't support LBR callstacks
Date: Wed,  6 Mar 2024 17:13:44 -0800	[thread overview]
Message-ID: <20240307011344.835640-4-seanjc@google.com> (raw)
In-Reply-To: <20240307011344.835640-1-seanjc@google.com>

Disable LBR virtualization if the CPU doesn't support callstacks, which
were introduced in HSW (see commit e9d7f7cd97c4 ("perf/x86/intel: Add
basic Haswell LBR call stack support"), as KVM unconditionally configures
the perf LBR event with PERF_SAMPLE_BRANCH_CALL_STACK, i.e. LBR
virtualization always fails on pre-HSW CPUs.

Simply disable LBR support on such CPUs, as it has never worked, i.e.
there is no risk of breaking an existing setup, and figuring out a way
to performantly context switch LBRs on old CPUs is not worth the effort.

Fixes: be635e34c284 ("KVM: vmx/pmu: Expose LBR_FMT in the MSR_IA32_PERF_CAPABILITIES")
Cc: Mingwei Zhang <mizhang@google.com>
Cc: Jim Mattson <jmattson@google.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
---
 arch/x86/kvm/vmx/vmx.c | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index 2a7cd66988a5..25a7652bee7c 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -7859,7 +7859,15 @@ static __init u64 vmx_get_perf_capabilities(void)
 
 	if (!cpu_feature_enabled(X86_FEATURE_ARCH_LBR)) {
 		x86_perf_get_lbr(&vmx_lbr_caps);
-		if (vmx_lbr_caps.nr)
+
+		/*
+		 * KVM requires LBR callstack support, as the overhead due to
+		 * context switching LBRs without said support is too high.
+		 * See intel_pmu_create_guest_lbr_event() for more info.
+		 */
+		if (!vmx_lbr_caps.has_callstack)
+			memset(&vmx_lbr_caps, 0, sizeof(vmx_lbr_caps));
+		else if (vmx_lbr_caps.nr)
 			perf_cap |= host_perf_cap & PMU_CAP_LBR_FMT;
 	}
 
-- 
2.44.0.278.ge034bb2e1d-goog

