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

[Sean Christopherson]

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@xxxxxxxxxx>
---
 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