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KVM: VMX: Disable LBR virtualization if the CPU doesn't support LBR callstacks

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Disable LBR virtualization if the CPU doesn't support callstacks, which
were introduced in HSW (see commit e9d7f7cd97 ("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: be635e34c2 ("KVM: vmx/pmu: Expose LBR_FMT in the MSR_IA32_PERF_CAPABILITIES")
Cc: Mingwei Zhang <mizhang@google.com>
Cc: Jim Mattson <jmattson@google.com>
Tested-by: Mingwei Zhang <mizhang@google.com>
Link: https://lore.kernel.org/r/20240307011344.835640-4-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>
This commit is contained in:
Sean Christopherson
2024-03-06 17:13:44 -08:00
parent 0d0b608650
commit bb9dc85908
1 changed files with 9 additions and 1 deletions
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10
arch/x86/kvm/vmx/vmx.c
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@@ -7877,7 +7877,15 @@ 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;
}