linux-kernel - Re: [PATCH 12/28] KVM: x86/mmu: Batch TLB flushes from TDP MMU for MMU notifier change

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Date:   Mon, 22 Nov 2021 13:45:05 -0800
From:   Ben Gardon <bgardon@...gle.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Hou Wenlong <houwenlong93@...ux.alibaba.com>
Subject: Re: [PATCH 12/28] KVM: x86/mmu: Batch TLB flushes from TDP MMU for
 MMU notifier change_spte

On Fri, Nov 19, 2021 at 8:51 PM Sean Christopherson <seanjc@...gle.com> wrote:
>
> Batch TLB flushes (with other MMUs) when handling ->change_spte()
> notifications in the TDP MMU.  The MMU notifier path in question doesn't
> allow yielding and correcty flushes before dropping mmu_lock.
>
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>

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

This seems very reasonable to me.

> ---
>  arch/x86/kvm/mmu/tdp_mmu.c | 13 ++++++-------
>  1 file changed, 6 insertions(+), 7 deletions(-)
>
> diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
> index 8e446ef03022..06b500fab248 100644
> --- a/arch/x86/kvm/mmu/tdp_mmu.c
> +++ b/arch/x86/kvm/mmu/tdp_mmu.c
> @@ -1182,13 +1182,12 @@ static bool set_spte_gfn(struct kvm *kvm, struct tdp_iter *iter,
>   */
>  bool kvm_tdp_mmu_set_spte_gfn(struct kvm *kvm, struct kvm_gfn_range *range)
>  {
> -       bool flush = kvm_tdp_mmu_handle_gfn(kvm, range, set_spte_gfn);
> -
> -       /* FIXME: return 'flush' instead of flushing here. */
> -       if (flush)
> -               kvm_flush_remote_tlbs_with_address(kvm, range->start, 1);
> -
> -       return false;
> +       /*
> +        * No need to handle the remote TLB flush under RCU protection, the
> +        * target SPTE _must_ be a leaf SPTE, i.e. cannot result in freeing a
> +        * shadow page.  See the WARN on pfn_changed in __handle_changed_spte().
> +        */
> +       return kvm_tdp_mmu_handle_gfn(kvm, range, set_spte_gfn);
>  }
>
>  /*
> --
> 2.34.0.rc2.393.gf8c9666880-goog
>