linux-kernel - Re: [PATCH v8 05/11] KVM: x86/mmu: Rearrange kvm_{test_,}age

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Message-ID: <CADrL8HXHy5j6QLU_7+jeFbdiddeAMg_-HAWwO391qkOTuXCuQQ@mail.gmail.com>
Date: Mon, 27 Jan 2025 11:58:11 -0800
From: James Houghton <jthoughton@...gle.com>
To: Sean Christopherson <seanjc@...gle.com>
Cc: Paolo Bonzini <pbonzini@...hat.com>, David Matlack <dmatlack@...gle.com>, 
	David Rientjes <rientjes@...gle.com>, Marc Zyngier <maz@...nel.org>, 
	Oliver Upton <oliver.upton@...ux.dev>, Wei Xu <weixugc@...gle.com>, Yu Zhao <yuzhao@...gle.com>, 
	Axel Rasmussen <axelrasmussen@...gle.com>, kvm@...r.kernel.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v8 05/11] KVM: x86/mmu: Rearrange kvm_{test_,}age_gfn

On Fri, Jan 10, 2025 at 2:59 PM Sean Christopherson <seanjc@...gle.com> wrote:
>
> On Tue, Nov 05, 2024, James Houghton wrote:
> > Reorder the TDP MMU check to be first for both kvm_test_age_gfn and
>
> () on functions, i.e. kvm_test_age_gfn().  That said, even better would be to
> avoid using the function names.  Let the patch itself communicate which functions
> are affected, and instead write the changelog as you would verbally communicate
> the change.
>
> > kvm_age_gfn. For kvm_test_age_gfn, this allows us to completely avoid
>
> No "us" or "we".
>
>
> > needing to grab the MMU lock when the TDP MMU reports that the page is
> > young.
>
> The changelog should make it clear that the patch actually does this, i.e. that
> there is a functional change beyond just changing the ordering.  Ooh, and that
> definitely needs to be captured in the shortlog.  I would even go so far as to
> say it should be the focal point of the shortlog.
>
> E.g. something like:
>
> KVM: x86/mmu: Skip shadow MMU test_young if TDP MMU reports page as young
>
> Reorder the processing of the TDP MMU versus the shadow MMU when aging
> SPTEs, and skip the shadow MMU entirely in the test-only case if the TDP
> MMU reports that the page is young, i.e. completely avoid taking mmu_lock
> if the TDP MMU SPTE is young.  Swap the order for the test-and-age helper
> as well for consistency.

Thanks, I think this is worded very clearly. Applied verbatim.

Noted your tips for future changelogs.