| Message ID | 20240307011344.835640-4-seanjc@google.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | KVM: VMX: Disable LBRs if CPU doesn't have callstacks | expand |
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 >
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; }
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(-)