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

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Message-ID: <Z4GmS48TBDetli-X@google.com>
Date: Fri, 10 Jan 2025 14:59:23 -0800
From: Sean Christopherson <seanjc@...gle.com>
To: James Houghton <jthoughton@...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 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.

> Do the same for kvm_age_gfn merely for consistency.