linux-kernel - [PATCH v3 8/8] KVM: x86/cpuid: Use fast return for cpuid "0xa" leaf when !enable

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Date:   Fri, 11 Nov 2022 18:26:45 +0800
From:   Like Xu <like.xu.linux@...il.com>
To:     Sean Christopherson <seanjc@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: [PATCH v3 8/8] KVM: x86/cpuid: Use fast return for cpuid "0xa" leaf when !enable_pmu

From: Like Xu <likexu@...cent.com>

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 <seanjc@...gle.com>
Signed-off-by: Like Xu <likexu@...cent.com>
---
 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