linux-kernel - Re: [PATCH] KVM: x86/mmu: Speed up slot_rmap_walk

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Message-ID: <CALzav=e6W2VSp=btmqTpQJ=3bH+Bw3D8sLApkTTvMMKAnw_LAw@mail.gmail.com>
Date:   Fri, 25 Mar 2022 16:52:45 -0700
From:   David Matlack <dmatlack@...gle.com>
To:     Vipin Sharma <vipinsh@...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 list <kvm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] KVM: x86/mmu: Speed up slot_rmap_walk_next for sparsely
 populated rmaps

On Fri, Mar 25, 2022 at 4:31 PM Vipin Sharma <vipinsh@...gle.com> wrote:
>
> Avoid calling handlers on empty rmap entries and skip to the next non
> empty rmap entry.
>
> Empty rmap entries are noop in handlers.
>
> Signed-off-by: Vipin Sharma <vipinsh@...gle.com>
> Suggested-by: Sean Christopherson <seanjc@...gle.com>
> Change-Id: I8abf0f4d82a2aae4c5d58b80bcc17ffc30785ffc

nit: Omit Change-Id tags from upstream commits.

> ---
>  arch/x86/kvm/mmu/mmu.c | 9 ++++++---
>  1 file changed, 6 insertions(+), 3 deletions(-)
>
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index 51671cb34fb6..f296340803ba 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -1499,11 +1499,14 @@ static bool slot_rmap_walk_okay(struct slot_rmap_walk_iterator *iterator)
>         return !!iterator->rmap;
>  }
>
> -static void slot_rmap_walk_next(struct slot_rmap_walk_iterator *iterator)
> +static noinline void

What is the reason to add noinline?


> +slot_rmap_walk_next(struct slot_rmap_walk_iterator *iterator)
>  {
> -       if (++iterator->rmap <= iterator->end_rmap) {
> +       while (++iterator->rmap <= iterator->end_rmap) {
>                 iterator->gfn += (1UL << KVM_HPAGE_GFN_SHIFT(iterator->level));
> -               return;
> +
> +               if (iterator->rmap->val)
> +                       return;
>         }
>
>         if (++iterator->level > iterator->end_level) {
>
> base-commit: c9b8fecddb5bb4b67e351bbaeaa648a6f7456912
> --
> 2.35.1.1021.g381101b075-goog
>