diff mbox series

[v4,07/12] KVM: x86/cpuid: Use fast return for cpuid "0xa" leaf when !enable_pmu

Message ID 20230214050757.9623-8-likexu@tencent.com
State New
Headers
Series KVM: x86: Add AMD Guest PerfMonV2 PMU support |

Commit Message

Like Xu Feb. 14, 2023, 5:07 a.m. UTC 
  From: Like Xu <likexu@tencent.com>

Add an explicit !enable_pmu check as relying on kvm_pmu_cap to be
zeroed isn't obvious. 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@google.com>
Signed-off-by: Like Xu <likexu@tencent.com>
---
 arch/x86/kvm/cpuid.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

Comments

Sean Christopherson April 6, 2023, 11:59 p.m. UTC | #1 
KVM: x86: Explicitly zero cpuid "0xa" leaf when PMU is disabled

On Tue, Feb 14, 2023, Like Xu wrote:
> From: Like Xu <likexu@tencent.com>
> 
> Add an explicit !enable_pmu check as relying on kvm_pmu_cap to be
> zeroed isn't obvious. 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.

Drop the performance blurb, as I stated multiple times, optimizing this code is
a non-goal.
diff mbox series

Patch

diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
index 2a9f1e200dbc..b0bb5f9f5307 100644
--- a/arch/x86/kvm/cpuid.c
+++ b/arch/x86/kvm/cpuid.c
@@ -944,7 +944,7 @@  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;
 		}