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

Ben Gardon
2022-02-28T23:18:15Z

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
>