2022-11-11 10:51:21

by Like Xu

Subject: [PATCH v3 8/8] KVM: x86/cpuid: Use fast return for cpuid "0xa" leaf when !enable_pmu

From: Like Xu <[email protected]>

Although when !enable_pmu, KVM will have zero-padded kvm_pmu_cap to do
subsequent cpuid leaf assignments, one extra branch instruction saves
a few subsequent zero-assignment instructions, speeding things up a bit.

Suggested-by: Sean Christopherson <[email protected]>
Signed-off-by: Like Xu <[email protected]>
---
arch/x86/kvm/cpuid.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
index df551fa66ccc..719290ff6d77 100644
--- a/arch/x86/kvm/cpuid.c
+++ b/arch/x86/kvm/cpuid.c
@@ -922,7 +922,7 @@ static inline int __do_cpuid_func(struct kvm_cpuid_array *array, u32 function)
union cpuid10_eax eax;
union cpuid10_edx edx;

- if (!static_cpu_has(X86_FEATURE_ARCH_PERFMON)) {
+ if (!enable_pmu || !static_cpu_has(X86_FEATURE_ARCH_PERFMON)) {
entry->eax = entry->ebx = entry->ecx = entry->edx = 0;
break;
}
--
2.38.1

2023-01-20 02:13:01

by Sean Christopherson

Subject: Re: [PATCH v3 8/8] KVM: x86/cpuid: Use fast return for cpuid "0xa" leaf when !enable_pmu

On Fri, Nov 11, 2022, Like Xu wrote:
> From: Like Xu <[email protected]>
>
> Although when !enable_pmu, KVM will have zero-padded kvm_pmu_cap to do
> subsequent cpuid leaf assignments, one extra branch instruction saves
> a few subsequent zero-assignment instructions, speeding things up a bit.

I doubt my motivation was performance, I'm guessing I suggested adding the explicit
!enable_pmu check because relying on kvm_pmu_cap to be zeroed isn't obvious.