linux-kernel - Re: [PATCH v8 06/11] KVM: x86/mmu: Only check gfn age in shadow MMU if indirect_shadow

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Message-ID: <CADrL8HVtogbCnS=K8-Jm22rkTfNKh4uFieXCbuknfpYJWg=pfQ@mail.gmail.com>
Date: Mon, 27 Jan 2025 11:58:27 -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 06/11] KVM: x86/mmu: Only check gfn age in shadow MMU
 if indirect_shadow_pages > 0

On Fri, Jan 10, 2025 at 3:05 PM Sean Christopherson <seanjc@...gle.com> wrote:
>
> On Tue, Nov 05, 2024, James Houghton wrote:
> > Optimize both kvm_age_gfn and kvm_test_age_gfn's interaction with the
> > shadow MMU by, rather than checking if our memslot has rmaps, check if
>
> No "our" (pronouns bad).
>
> > there are any indirect_shadow_pages at all.
>
> Again, use wording that is more conversational.  I also think it's worthwhile to
> call out when this optimization is helpful.  E.g.
>
> When aging SPTEs and the TDP MMU is enabled, process the shadow MMU if and
> only if the VM has at least one shadow page, as opposed to checking if the
> VM has rmaps.  Checking for rmaps will effectively yield a false positive
> if the VM ran nested TDP VMs in the past, but is not currently doing so.

Applied verbatim. Thanks.

> > Signed-off-by: James Houghton <jthoughton@...gle.com>
> > ---
> >  arch/x86/kvm/mmu/mmu.c | 9 +++++++--
> >  1 file changed, 7 insertions(+), 2 deletions(-)
> >
> > diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> > index 793565a3a573..125d4c3ccceb 100644
> > --- a/arch/x86/kvm/mmu/mmu.c
> > +++ b/arch/x86/kvm/mmu/mmu.c
> > @@ -1582,6 +1582,11 @@ static bool kvm_rmap_age_gfn_range(struct kvm *kvm,
> >       return young;
> >  }
> >
> > +static bool kvm_has_shadow_mmu_sptes(struct kvm *kvm)
>
> I think this should be kvm_may_have_shadow_mmu_sptes(), or something along those
> lines that makes it clear the check is imprecise.  E.g. to avoid someone thinking
> that KVM is guaranteed to have shadow MMU SPTEs if it returns true.

Sounds good to me. Renamed.