linux-kernel - Re: [Patch v2 2/5] KVM: x86/mmu: Optimize SPTE change flow for clear-dirty-log

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Message-ID: <CAHVum0ffSwm62Tv9=HbQbMy42iq9Z4skMp1FxTVTothVmTgM5w@mail.gmail.com>
Date:   Tue, 7 Feb 2023 09:41:50 -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 2/5] KVM: x86/mmu: Optimize SPTE change flow for clear-dirty-log

On Mon, Feb 6, 2023 at 3:53 PM David Matlack <dmatlack@...gle.com> wrote:
>
> On Fri, Feb 3, 2023 at 11:28 AM Vipin Sharma <vipinsh@...gle.com> wrote:
> >
> > diff --git a/arch/x86/kvm/mmu/tdp_iter.h b/arch/x86/kvm/mmu/tdp_iter.h
> > index 30a52e5e68de..21046b34f94e 100644
> > --- a/arch/x86/kvm/mmu/tdp_iter.h
> > +++ b/arch/x86/kvm/mmu/tdp_iter.h
> > @@ -121,4 +121,17 @@ void tdp_iter_start(struct tdp_iter *iter, struct kvm_mmu_page *root,
> >  void tdp_iter_next(struct tdp_iter *iter);
> >  void tdp_iter_restart(struct tdp_iter *iter);
> >
> > +static inline u64 kvm_tdp_mmu_clear_spte_bit(struct tdp_iter *iter, u64 mask)
> > +{
> > +       atomic64_t *sptep;
> > +
> > +       if (kvm_tdp_mmu_spte_has_volatile_bits(iter->old_spte, iter->level)) {
> > +               sptep = (atomic64_t *)rcu_dereference(iter->sptep);
> > +               return (u64)atomic64_fetch_and(~mask, sptep);
>
> I think you can just set iter->old_spte here and drop the return value?
>

I can do this. However, my intention was to keep the return contract
similar to kvm_tdp_mmu_write_spte(). On second thought, should I make
this function signature similar to kvm_tdp_mmu_write_spte() just to be
consistent?



> > +       }
> > +
> > +       __kvm_tdp_mmu_write_spte(iter->sptep, iter->old_spte & ~mask);
> > +       return iter->old_spte;
> > +}