linux-kernel - Re: [Patch v2 3/5] KVM: x86/mmu: Optimize SPTE change for aging gfn range

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Date:   Tue, 7 Feb 2023 09:50:15 -0800
From:   Vipin Sharma <vipinsh@...gle.com>
To:     David Matlack <dmatlack@...gle.com>
Cc:     seanjc@...gle.com, pbonzini@...hat.com, bgardon@...gle.com,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [Patch v2 3/5] KVM: x86/mmu: Optimize SPTE change for aging gfn range

On Mon, Feb 6, 2023 at 3:57 PM David Matlack <dmatlack@...gle.com> wrote:
>
> On Fri, Feb 03, 2023 at 11:28:20AM -0800, Vipin Sharma wrote:
> > No need to check all of the conditions in __handle_changed_spte(). Aging
> > a gfn range implies resetting access bit or marking spte for access
> > tracking.
> >
> > Use atomic operation to only reset those bits. This avoids checking many
> > conditions in __handle_changed_spte() API. Also, clean up code by
> > removing dead code and API parameters.
>
> Suggest splitting out the dead code cleanup to make it easier to review.
>

Sounds good.

> >
> > Signed-off-by: Vipin Sharma <vipinsh@...gle.com>
> > ---
> >  arch/x86/kvm/mmu/tdp_mmu.c | 68 ++++++++++++++------------------------
> >  1 file changed, 25 insertions(+), 43 deletions(-)
> >
> > diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
> > index 83f15052aa6c..18630a06fa1f 100644
> > --- a/arch/x86/kvm/mmu/tdp_mmu.c
> > +++ b/arch/x86/kvm/mmu/tdp_mmu.c
> > @@ -1251,32 +1228,37 @@ static __always_inline bool kvm_tdp_mmu_handle_gfn(struct kvm *kvm,
> >  /*
> >   * Mark the SPTEs range of GFNs [start, end) unaccessed and return non-zero
> >   * if any of the GFNs in the range have been accessed.
> > + *
> > + * No need to mark corresponding PFN as accessed as this call is coming from
> > + * MMU notifier for that page via HVA.
>
> Thanks for adding this comment.
>
> Can you just extend it to mention that the information is passed via the
> return value? e.g.
>
>  * No need to mark corresponding PFN as accessed as this call is coming
>  * from the clear_young() or clear_flush_young() notifier, which uses
>  * the return value to determine if the page has been accessed.
>

Sure.