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

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Message-ID: <CALzav=d9h-fVgdsK138m74a_qTyay9cprcbdWAJk4GJtw4p6tg@mail.gmail.com>
Date:   Mon, 6 Feb 2023 15:53:21 -0800
From:   David Matlack <dmatlack@...gle.com>
To:     Vipin Sharma <vipinsh@...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 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?

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