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

Sean Christopherson
2022-02-26T00:16:29Z

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>
---
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