linux-kernel - Re: [PATCH 10/28] KVM: x86/mmu: Allow yielding when zapping GFNs for defunct TDP MMU root

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Date:   Mon, 22 Nov 2021 13:30:42 -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 10/28] KVM: x86/mmu: Allow yielding when zapping GFNs for
 defunct TDP MMU root

On Fri, Nov 19, 2021 at 8:51 PM Sean Christopherson <seanjc@...gle.com> wrote:
>
> Allow yielding when zapping SPTEs for a defunct TDP MMU root.  Yielding
> is safe from a TDP perspective, as the root is unreachable.  The only
> potential danger is putting a root from a non-preemptible context, and
> KVM currently does not do so.
>
> Yield-unfriendly iteration uses for_each_tdp_mmu_root(), which doesn't
> take a reference to each root (it requires mmu_lock be held for the
> entire duration of the walk).
>
> tdp_mmu_next_root() is used only by the yield-friendly iterator.
>
> kvm_tdp_mmu_zap_invalidated_roots() is explicitly yield friendly.
>
> kvm_mmu_free_roots() => mmu_free_root_page() is a much bigger fan-out,
> but is still yield-friendly in all call sites, as all callers can be
> traced back to some combination of vcpu_run(), kvm_destroy_vm(), and/or
> kvm_create_vm().
>
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>

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

I'm glad to see this fixed. I assume we don't usually hit this in
testing because most of the teardown happens in the zap-all path when
we unregister for MMU notifiers and actually deleting a fully
populated root while the VM is running is pretty rare.

> ---
>  arch/x86/kvm/mmu/tdp_mmu.c | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
> index 3086c6dc74fb..138c7dc41d2c 100644
> --- a/arch/x86/kvm/mmu/tdp_mmu.c
> +++ b/arch/x86/kvm/mmu/tdp_mmu.c
> @@ -79,6 +79,11 @@ static void tdp_mmu_free_sp_rcu_callback(struct rcu_head *head)
>         tdp_mmu_free_sp(sp);
>  }
>
> +/*
> + * Note, putting a root might sleep, i.e. the caller must have IRQs enabled and
> + * must not explicitly disable preemption (it will be disabled by virtue of
> + * holding mmu_lock, hence the lack of a might_sleep()).
> + */
>  void kvm_tdp_mmu_put_root(struct kvm *kvm, struct kvm_mmu_page *root,
>                           bool shared)
>  {
> @@ -101,7 +106,7 @@ void kvm_tdp_mmu_put_root(struct kvm *kvm, struct kvm_mmu_page *root,
>          * intermediate paging structures, that may be zapped, as such entries
>          * are associated with the ASID on both VMX and SVM.
>          */
> -       (void)zap_gfn_range(kvm, root, 0, -1ull, false, false, shared);
> +       (void)zap_gfn_range(kvm, root, 0, -1ull, true, false, shared);
>
>         call_rcu(&root->rcu_head, tdp_mmu_free_sp_rcu_callback);
>  }
> --
> 2.34.0.rc2.393.gf8c9666880-goog
>