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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <ZC9cxD6lLcF5tYHF@google.com>
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.