Commit bb9dc859 authored by Sean Christopherson's avatar Sean Christopherson
Browse files

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



Disable LBR virtualization if the CPU doesn't support callstacks, which
were introduced in HSW (see commit e9d7f7cd ("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: be635e34 ("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: default avatarMingwei Zhang <mizhang@google.com>
Link: https://lore.kernel.org/r/20240307011344.835640-4-seanjc@google.com


Signed-off-by: default avatarSean Christopherson <seanjc@google.com>
parent 0d0b6086

arch/x86/kvm/vmx/vmx.c

+9 −1
Original line number Diff line number Diff line
@@ -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;

	}