linux-kernel - Re: [PATCH 21/28] KVM: x86/mmu: Add TDP MMU helper to zap a root

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Date:   Mon, 22 Nov 2021 14:54:57 -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 21/28] KVM: x86/mmu: Add TDP MMU helper to zap a root

On Fri, Nov 19, 2021 at 8:51 PM Sean Christopherson <seanjc@...gle.com> wrote:
>
> Add a small wrapper to handle zapping a specific root.  For now, it's
> little more than syntactic sugar, but in the future it will become a
> unique flow with rules specific to zapping an unreachable root.
>
> No functional change intended.
>
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> ---
>  arch/x86/kvm/mmu/tdp_mmu.c | 11 +++++++++--
>  1 file changed, 9 insertions(+), 2 deletions(-)
>
> diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
> index 9449cb5baf0b..31fb622249e5 100644
> --- a/arch/x86/kvm/mmu/tdp_mmu.c
> +++ b/arch/x86/kvm/mmu/tdp_mmu.c
> @@ -79,11 +79,18 @@ static void tdp_mmu_free_sp_rcu_callback(struct rcu_head *head)
>         tdp_mmu_free_sp(sp);
>  }
>
> +static bool tdp_mmu_zap_root(struct kvm *kvm, struct kvm_mmu_page *root,
> +                            bool shared)
> +{
> +       return zap_gfn_range(kvm, root, 0, -1ull, true, false, shared);

Total aside:
Remembering the order of these three boolean parameters through all
these functions drives me nuts.
It'd be really nice to put them into a neat, reusable struct that tracks:
MMU lock mode (read / write / none)
If yielding is okay
If the TLBs are dirty and need to be flushed

I don't know when I'll have time to do that refactor, but it would
make this code so much more sensible.

> +}
> +
>  /*
>   * 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)
>  {
> @@ -118,7 +125,7 @@ void kvm_tdp_mmu_put_root(struct kvm *kvm, struct kvm_mmu_page *root,
>          * should have been zapped by kvm_tdp_mmu_zap_invalidated_roots(), and
>          * inserting new SPTEs under an invalid root is a KVM bug.
>          */
> -       if (zap_gfn_range(kvm, root, 0, -1ull, true, false, shared))
> +       if (tdp_mmu_zap_root(kvm, root, shared))
>                 WARN_ON_ONCE(root->role.invalid);
>
>         call_rcu(&root->rcu_head, tdp_mmu_free_sp_rcu_callback);
> @@ -923,7 +930,7 @@ void kvm_tdp_mmu_zap_invalidated_roots(struct kvm *kvm,
>                  * will still flush on yield, but that's a minor performance
>                  * blip and not a functional issue.
>                  */
> -               (void)zap_gfn_range(kvm, root, 0, -1ull, true, false, true);
> +               (void)tdp_mmu_zap_root(kvm, root, true);
>                 kvm_tdp_mmu_put_root(kvm, root, true);
>         }
>  }
> --
> 2.34.0.rc2.393.gf8c9666880-goog
>