KVM: leverage change to adjust slots->used_slots in update_memslots()

update_memslots() is only called by __kvm_set_memory_region(), in which
"change" is calculated and indicates how to adjust slots->used_slots

  * increase by one if it is KVM_MR_CREATE
  * decrease by one if it is KVM_MR_DELETE
  * not change for others

This patch adjusts slots->used_slots in update_memslots() based on "change"
value instead of re-calculate those states again.

Signed-off-by: Wei Yang <richard.weiyang@gmail.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

virt/kvm/kvm_main.c

@@ -805,20 +805,25 @@ static int kvm_create_dirty_bitmap(struct kvm_memory_slot *memslot)
  * sorted array and known changed memslot position.
  */
 static void update_memslots(struct kvm_memslots *slots,
-			    struct kvm_memory_slot *new)
+			    struct kvm_memory_slot *new,
+			    enum kvm_mr_change change)
 {
 	int id = new->id;
 	int i = slots->id_to_index[id];
 	struct kvm_memory_slot *mslots = slots->memslots;
 
 	WARN_ON(mslots[i].id != id);
-	if (!new->npages) {
-		WARN_ON(!mslots[i].npages);
-		if (mslots[i].npages)
-			slots->used_slots--;
-	} else {
-		if (!mslots[i].npages)
-			slots->used_slots++;
+	switch (change) {
+	case KVM_MR_CREATE:
+		slots->used_slots++;
+		WARN_ON(mslots[i].npages || !new->npages);
+		break;
+	case KVM_MR_DELETE:
+		slots->used_slots--;
+		WARN_ON(new->npages || !mslots[i].npages);
+		break;
+	default:
+		break;
 	}
 
 	while (i < KVM_MEM_SLOTS_NUM - 1 &&
@@ -1054,7 +1059,7 @@ int __kvm_set_memory_region(struct kvm *kvm,
 		memset(&new.arch, 0, sizeof(new.arch));
 	}
 
-	update_memslots(slots, &new);
+	update_memslots(slots, &new, change);
 	old_memslots = install_new_memslots(kvm, as_id, slots);
 
 	kvm_arch_commit_memory_region(kvm, mem, &old, &new, change);