linux-kernel - Re: [PATCH v3 14/28] KVM: x86/mmu: Skip remote TLB flush when zapping all of TDP MMU

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Message-ID: <CANgfPd9yGgXxT_Mue9DKpmtsCSrUQHCME7GcqYScEwxFwetimQ@mail.gmail.com>
Date:   Mon, 28 Feb 2022 16:19:44 -0800
From:   Ben Gardon <bgardon@...gle.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Christian Borntraeger <borntraeger@...ux.ibm.com>,
        Janosch Frank <frankja@...ux.ibm.com>,
        Claudio Imbrenda <imbrenda@...ux.ibm.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        David Hildenbrand <david@...hat.com>,
        kvm <kvm@...r.kernel.org>, LKML <linux-kernel@...r.kernel.org>,
        David Matlack <dmatlack@...gle.com>,
        Mingwei Zhang <mizhang@...gle.com>
Subject: Re: [PATCH v3 14/28] KVM: x86/mmu: Skip remote TLB flush when zapping
 all of TDP MMU

On Fri, Feb 25, 2022 at 4:16 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>

Reviewed-by: Ben Gardon <bgardon@...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 c231b60e1726..87706e9cc6f3 100644
> --- a/arch/x86/kvm/mmu/tdp_mmu.c
> +++ b/arch/x86/kvm/mmu/tdp_mmu.c
> @@ -874,14 +874,15 @@ 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 zaps 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.
> +        */
>         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);
>  }
>
>  /*
> --
> 2.35.1.574.g5d30c73bfb-goog
>