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

Mingwei Zhang
2024-03-18T22:51:07Z

On Wed, Mar 06, 2024, Sean Christopherson wrote:
> 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>
Tested-by: Mingwei Zhang <mizhang@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
>