KVM: x86/mmu: Make TDP MMU root refcount atomic

In order to parallelize more operations for the TDP MMU, make the
refcount on TDP MMU roots atomic, so that a future patch can allow
multiple threads to take a reference on the root concurrently, while
holding the MMU lock in read mode.
Signed-off-by: Ben Gardon <bgardon@google.com>
Message-Id: <20210401233736.638171-7-bgardon@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/mmu/mmu_internal.h

@@ -50,7 +50,11 @@ struct kvm_mmu_page {
 	u64 *spt;
 	/* hold the gfn of each spte inside spt */
 	gfn_t *gfns;
-	int root_count;          /* Currently serving as active root */
+	/* Currently serving as active root */
+	union {
+		int root_count;
+		refcount_t tdp_mmu_root_count;
+	};
 	unsigned int unsync_children;
 	struct kvm_rmap_head parent_ptes; /* rmap pointers to parent sptes */
 	DECLARE_BITMAP(unsync_child_bitmap, 512);

arch/x86/kvm/mmu/tdp_mmu.c

@@ -56,7 +56,7 @@ void kvm_tdp_mmu_put_root(struct kvm *kvm, struct kvm_mmu_page *root)
 
 	lockdep_assert_held_write(&kvm->mmu_lock);
 
-	if (--root->root_count)
+	if (!refcount_dec_and_test(&root->tdp_mmu_root_count))
 		return;
 
 	WARN_ON(!root->tdp_mmu_page);
@@ -168,7 +168,7 @@ hpa_t kvm_tdp_mmu_get_vcpu_root_hpa(struct kvm_vcpu *vcpu)
 	}
 
 	root = alloc_tdp_mmu_page(vcpu, 0, vcpu->arch.mmu->shadow_root_level);
-	root->root_count = 1;
+	refcount_set(&root->tdp_mmu_root_count, 1);
 
 	list_add(&root->link, &kvm->arch.tdp_mmu_roots);
 

arch/x86/kvm/mmu/tdp_mmu.h

@@ -10,10 +10,14 @@ hpa_t kvm_tdp_mmu_get_vcpu_root_hpa(struct kvm_vcpu *vcpu);
 static inline void kvm_tdp_mmu_get_root(struct kvm *kvm,
 					struct kvm_mmu_page *root)
 {
-	BUG_ON(!root->root_count);
-	lockdep_assert_held(&kvm->mmu_lock);
+	lockdep_assert_held_write(&kvm->mmu_lock);
 
-	++root->root_count;
+	/*
+	 * This should never fail since roots are removed from the roots
+	 * list under the MMU write lock when their reference count falls
+	 * to zero.
+	 */
+	refcount_inc_not_zero(&root->tdp_mmu_root_count);
 }
 
 void kvm_tdp_mmu_put_root(struct kvm *kvm, struct kvm_mmu_page *root);