linux-kernel - Re: [PATCH v4 07/12] KVM: x86/cpuid: Use fast return for cpuid "0xa" leaf when !enable

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Date:   Thu, 6 Apr 2023 16:59:00 -0700
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 v4 07/12] KVM: x86/cpuid: Use fast return for cpuid "0xa"
 leaf when !enable_pmu

KVM: x86: Explicitly zero cpuid "0xa" leaf when PMU is disabled

On Tue, Feb 14, 2023, Like Xu wrote:
> From: Like Xu <likexu@...cent.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.