linux-kernel - Re: [PATCH v2 1/3] KVM: x86/mmu: Remove redundant spte present check in mmu_set

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Date:   Mon, 26 Jul 2021 13:23:34 -0700
From:   Ben Gardon <bgardon@...gle.com>
To:     Mingwei Zhang <mizhang@...gle.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm <kvm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Jing Zhang <jingzhangos@...gle.com>
Subject: Re: [PATCH v2 1/3] KVM: x86/mmu: Remove redundant spte present check
 in mmu_set_spte

On Mon, Jul 26, 2021 at 10:54 AM Mingwei Zhang <mizhang@...gle.com> wrote:
>
> Drop an unnecessary is_shadow_present_pte() check when updating the rmaps
> after installing a non-MMIO SPTE.  set_spte() is used only to create
> shadow-present SPTEs, e.g. MMIO SPTEs are handled early on, mmu_set_spte()
> runs with mmu_lock held for write, i.e. the SPTE can't be zapped between
> writing the SPTE and updating the rmaps.
>
> Opportunistically combine the "new SPTE" logic for large pages and rmaps.
>
> No functional change intended.
>
> Suggested-by: Ben Gardon <bgardon@...gle.com>
> Signed-off-by: Mingwei Zhang <mizhang@...gle.com>

Reviewed-by: Ben Gardon <bgardon@...gle.com>

> ---
>  arch/x86/kvm/mmu/mmu.c | 14 ++++++--------
>  1 file changed, 6 insertions(+), 8 deletions(-)
>
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index b888385d1933..442cc554ebd6 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -2690,15 +2690,13 @@ static int mmu_set_spte(struct kvm_vcpu *vcpu, u64 *sptep,
>
>         pgprintk("%s: setting spte %llx\n", __func__, *sptep);
>         trace_kvm_mmu_set_spte(level, gfn, sptep);
> -       if (!was_rmapped && is_large_pte(*sptep))
> -               ++vcpu->kvm->stat.lpages;
>
> -       if (is_shadow_present_pte(*sptep)) {
> -               if (!was_rmapped) {
> -                       rmap_count = rmap_add(vcpu, sptep, gfn);
> -                       if (rmap_count > RMAP_RECYCLE_THRESHOLD)
> -                               rmap_recycle(vcpu, sptep, gfn);
> -               }
> +       if (!was_rmapped) {
> +               if (is_large_pte(*sptep))
> +                       ++vcpu->kvm->stat.lpages;
> +               rmap_count = rmap_add(vcpu, sptep, gfn);
> +               if (rmap_count > RMAP_RECYCLE_THRESHOLD)
> +                       rmap_recycle(vcpu, sptep, gfn);
>         }
>
>         return ret;
> --
> 2.32.0.432.gabb21c7263-goog
>