linux-kernel - Re: [V4 PATCH 2/4] KVM: selftests: x86: Add variables to store cpu type

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Message-ID: <CAGtprH981oz8VVjMHnD_PSmYLF1SmiVERXkgF45-JmErSAzRyQ@mail.gmail.com>
Date:   Tue, 10 Jan 2023 16:13:17 -0800
From:   Vishal Annapurve <vannapurve@...gle.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     x86@...nel.org, kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-kselftest@...r.kernel.org, pbonzini@...hat.com,
        shuah@...nel.org, bgardon@...gle.com, oupton@...gle.com,
        peterx@...hat.com, vkuznets@...hat.com, dmatlack@...gle.com
Subject: Re: [V4 PATCH 2/4] KVM: selftests: x86: Add variables to store cpu type

On Mon, Jan 9, 2023 at 10:13 AM Sean Christopherson <seanjc@...gle.com> wrote:
>
> In shortlogs and changelogs, try to provide a synopsis of the change, not a
> literal description of the change.  As suggested in the previous patch, this:
>
>   KVM: selftests: Cache host CPU vendor (AMD vs. Intel)
>
> is more precise (vendor instead of "cpu type") and hints at the intent (caching
> the information), whereas this doesn't capture the vendor part, nor does it provide
> any hint whatsoever as to (a) how the variables will be used or (b) why we want to
> add variables to store
>
>   KVM: selftests: x86: Add variables to store cpu type
>
> On Wed, Dec 28, 2022, Vishal Annapurve wrote:
> > Add variables to hold the cpu vendor type that are initialized early
> > during the selftest setup and later synced to guest vm post VM creation.
> >
> > These variables will be used in later patches to avoid querying CPU
> > type multiple times.
>
> Performance is a happy bonus, it is not the main reason for caching.  The main
> reason for caching is so that the guest can select the native hypercall instruction
> without having to make assumptions about guest vs. host CPUID information.

Ack, that makes sense.