KVM: x86/mmu: Revert "KVM: x86/mmu: Remove is_obsolete() call"

Now that the fast invalidate mechanism has been reintroduced, restore
the performance tweaks for fast invalidation that existed prior to its
removal.

Paraphrasing the original changelog (commit 5ff05683 was itself a
partial revert):

  Don't force reloading the remote mmu when zapping an obsolete page, as
  a MMU_RELOAD request has already been issued by kvm_mmu_zap_all_fast()
  immediately after incrementing mmu_valid_gen, i.e. after marking pages
  obsolete.

This reverts commit 5ff05683.
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/mmu.c

@@ -2752,7 +2752,12 @@ static bool __kvm_mmu_prepare_zap_page(struct kvm *kvm,
 	} else {
 		list_move(&sp->link, &kvm->arch.active_mmu_pages);
 
-		if (!sp->role.invalid)
+		/*
+		 * Obsolete pages cannot be used on any vCPUs, see the comment
+		 * in kvm_mmu_zap_all_fast().  Note, is_obsolete_sp() also
+		 * treats invalid shadow pages as being obsolete.
+		 */
+		if (!is_obsolete_sp(kvm, sp))
 			kvm_reload_remote_mmus(kvm);
 	}
 
@@ -5675,7 +5680,6 @@ int kvm_mmu_create(struct kvm_vcpu *vcpu)
 static void kvm_zap_obsolete_pages(struct kvm *kvm)
 {
 	struct kvm_mmu_page *sp, *node;
-	LIST_HEAD(invalid_list);
 	int nr_zapped, batch = 0;
 
 restart:
@@ -5708,8 +5712,8 @@ static void kvm_zap_obsolete_pages(struct kvm *kvm)
 			goto restart;
 		}
 
-		if (__kvm_mmu_prepare_zap_page(kvm, sp, &invalid_list,
-					       &nr_zapped)) {
+		if (__kvm_mmu_prepare_zap_page(kvm, sp,
+				&kvm->arch.zapped_obsolete_pages, &nr_zapped)) {
 			batch += nr_zapped;
 			goto restart;
 		}
@@ -5720,7 +5724,7 @@ static void kvm_zap_obsolete_pages(struct kvm *kvm)
 	 * KVM is not in the middle of a lockless shadow page table walk, which
 	 * may reference the pages.
 	 */
-	kvm_mmu_commit_zap_page(kvm, &invalid_list);
+	kvm_mmu_commit_zap_page(kvm, &kvm->arch.zapped_obsolete_pages);
 }
 
 /*
@@ -5752,6 +5756,11 @@ static void kvm_mmu_zap_all_fast(struct kvm *kvm)
 	spin_unlock(&kvm->mmu_lock);
 }
 
+static bool kvm_has_zapped_obsolete_pages(struct kvm *kvm)
+{
+	return unlikely(!list_empty_careful(&kvm->arch.zapped_obsolete_pages));
+}
+
 static void kvm_mmu_invalidate_zap_pages_in_memslot(struct kvm *kvm,
 			struct kvm_memory_slot *slot,
 			struct kvm_page_track_notifier_node *node)
@@ -6022,16 +6031,24 @@ mmu_shrink_scan(struct shrinker *shrink, struct shrink_control *sc)
 		 * want to shrink a VM that only started to populate its MMU
 		 * anyway.
 		 */
-		if (!kvm->arch.n_used_mmu_pages)
+		if (!kvm->arch.n_used_mmu_pages &&
+		    !kvm_has_zapped_obsolete_pages(kvm))
 			continue;
 
 		idx = srcu_read_lock(&kvm->srcu);
 		spin_lock(&kvm->mmu_lock);
 
+		if (kvm_has_zapped_obsolete_pages(kvm)) {
+			kvm_mmu_commit_zap_page(kvm,
+			      &kvm->arch.zapped_obsolete_pages);
+			goto unlock;
+		}
+
 		if (prepare_zap_oldest_mmu_page(kvm, &invalid_list))
 			freed++;
 		kvm_mmu_commit_zap_page(kvm, &invalid_list);
 
+unlock:
 		spin_unlock(&kvm->mmu_lock);
 		srcu_read_unlock(&kvm->srcu, idx);
 

arch/x86/kvm/x86.c

@@ -9425,6 +9425,7 @@ int kvm_arch_init_vm(struct kvm *kvm, unsigned long type)
 
 	INIT_HLIST_HEAD(&kvm->arch.mask_notifier_list);
 	INIT_LIST_HEAD(&kvm->arch.active_mmu_pages);
+	INIT_LIST_HEAD(&kvm->arch.zapped_obsolete_pages);
 	INIT_LIST_HEAD(&kvm->arch.assigned_dev_head);
 	atomic_set(&kvm->arch.noncoherent_dma_count, 0);