Re: [PATCH v3 05/28] KVM: x86/mmu: Document that zapping invalidated roots doesn't need to flush

[Ben Gardon <bgardon@google.com>]

On Fri, Feb 25, 2022 at 4:16 PM Sean Christopherson <seanjc@google.com> wrote:
>
> Remove the misleading flush "handling" when zapping invalidated TDP MMU
> roots, and document that flushing is unnecessary for all flavors of MMUs
> when zapping invalid/obsolete roots/pages.  The "handling" in the TDP MMU
> is dead code, as zap_gfn_range() is called with shared=true, in which
> case it will never return true due to the flushing being handled by
> tdp_mmu_zap_spte_atomic().
>
> No functional change intended.
>
> Signed-off-by: Sean Christopherson <seanjc@google.com>

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

> ---
>  arch/x86/kvm/mmu/mmu.c     | 10 +++++++---
>  arch/x86/kvm/mmu/tdp_mmu.c | 15 ++++++++++-----
>  2 files changed, 17 insertions(+), 8 deletions(-)
>
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index 5a931c89d27b..1c4b84e80841 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -5615,9 +5615,13 @@ static void kvm_zap_obsolete_pages(struct kvm *kvm)
>         }
>
>         /*
> -        * Trigger a remote TLB flush before freeing the page tables to ensure
> -        * KVM is not in the middle of a lockless shadow page table walk, which
> -        * may reference the pages.
> +        * Kick all vCPUs (via remote TLB flush) before freeing the page tables
> +        * to ensure KVM is not in the middle of a lockless shadow page table
> +        * walk, which may reference the pages.  The remote TLB flush itself is
> +        * not required and is simply a convenient way to kick vCPUs as needed.
> +        * KVM performs a local TLB flush when allocating a new root (see
> +        * kvm_mmu_load()), and the reload in the caller ensure no vCPUs are
> +        * running with an obsolete MMU.
>          */
>         kvm_mmu_commit_zap_page(kvm, &kvm->arch.zapped_obsolete_pages);
>  }
> diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
> index e35bd88d92fd..5994db5d5226 100644
> --- a/arch/x86/kvm/mmu/tdp_mmu.c
> +++ b/arch/x86/kvm/mmu/tdp_mmu.c
> @@ -843,12 +843,20 @@ void kvm_tdp_mmu_zap_all(struct kvm *kvm)
>  void kvm_tdp_mmu_zap_invalidated_roots(struct kvm *kvm)
>  {
>         struct kvm_mmu_page *root;
> -       bool flush = false;
>
>         lockdep_assert_held_read(&kvm->mmu_lock);
>
>         for_each_invalid_tdp_mmu_root_yield_safe(kvm, root) {
> -               flush = zap_gfn_range(kvm, root, 0, -1ull, true, flush, true);
> +               /*
> +                * A TLB flush is unnecessary, invalidated roots are guaranteed
> +                * to be unreachable by the guest (see kvm_tdp_mmu_put_root()
> +                * for more details), and unlike the legacy MMU, no vCPU kick
> +                * is needed to play nice with lockless shadow walks as the TDP
> +                * MMU protects its paging structures via RCU.  Note, zapping
> +                * 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);
>
>                 /*
>                  * Put the reference acquired in kvm_tdp_mmu_invalidate_roots().
> @@ -856,9 +864,6 @@ void kvm_tdp_mmu_zap_invalidated_roots(struct kvm *kvm)
>                  */
>                 kvm_tdp_mmu_put_root(kvm, root, true);
>         }
> -
> -       if (flush)
> -               kvm_flush_remote_tlbs(kvm);
>  }
>
>  /*
> --
> 2.35.1.574.g5d30c73bfb-goog
>