linux-kernel - Re: [PATCH 22/28] KVM: x86/mmu: Skip remote TLB flush when zapping all of TDP MMU

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Date:   Mon, 22 Nov 2021 15:00:17 -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 22/28] KVM: x86/mmu: Skip remote TLB flush when zapping
 all of TDP MMU

On Fri, Nov 19, 2021 at 8:51 PM Sean Christopherson <seanjc@...gle.com> wrote:
>
> Don't flush the TLBs when zapping all TDP MMU pages, as the only time KVM
> uses the slow version of "zap everything" is when the VM is being
> destroyed or the owning mm has exited.  In either case, KVM_RUN is
> unreachable for the VM, i.e. the guest TLB entries cannot be consumed.
>
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> ---
>  arch/x86/kvm/mmu/tdp_mmu.c | 11 ++++++-----
>  1 file changed, 6 insertions(+), 5 deletions(-)
>
> diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
> index 31fb622249e5..e5401f0efe8e 100644
> --- a/arch/x86/kvm/mmu/tdp_mmu.c
> +++ b/arch/x86/kvm/mmu/tdp_mmu.c
> @@ -888,14 +888,15 @@ bool __kvm_tdp_mmu_zap_gfn_range(struct kvm *kvm, int as_id, gfn_t start,
>
>  void kvm_tdp_mmu_zap_all(struct kvm *kvm)
>  {
> -       bool flush = false;
>         int i;
>
> +       /*
> +        * A TLB flush is unnecessary, KVM's zap everything if and only the VM
> +        * is being destroyed or the userspace VMM has exited.  In both cases,
> +        * KVM_RUN is unreachable, i.e. no vCPUs will ever service the request.
> +        */

Nit: Suggest:
       /*
        * A TLB flush is unnecessary. KVM's zap_all is used if and
only if the VM
        * is being destroyed or the userspace VMM has exited.  In both cases,
        * the vCPUs are not running and will never run again, so their
TLB state doesn't matter.
        */

>         for (i = 0; i < KVM_ADDRESS_SPACE_NUM; i++)
> -               flush = kvm_tdp_mmu_zap_gfn_range(kvm, i, 0, -1ull, flush);
> -
> -       if (flush)
> -               kvm_flush_remote_tlbs(kvm);
> +               (void)kvm_tdp_mmu_zap_gfn_range(kvm, i, 0, -1ull, false);
>  }
>
>  /*
> --
> 2.34.0.rc2.393.gf8c9666880-goog
>