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

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Date:   Fri, 20 Jan 2023 01:11:54 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Like Xu <like.xu.linux@...il.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
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 <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.

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.