KVM: set_memory_region: Don't check for overlaps unless we create or move a slot

Don't need the check for deleting an existing slot or just modifiying
the flags.
Reviewed-by: NMarcelo Tosatti <mtosatti@redhat.com>
Signed-off-by: NTakuya Yoshikawa <yoshikawa_takuya_b1@lab.ntt.co.jp>
Signed-off-by: NGleb Natapov <gleb@redhat.com>

virt/kvm/kvm_main.c

@@ -782,6 +782,7 @@ int __kvm_set_memory_region(struct kvm *kvm,
 	if (!npages && !old.npages)
 		goto out;
 
+	if ((npages && !old.npages) || (base_gfn != old.base_gfn)) {
 		/* Check for overlaps */
 		r = -EEXIST;
 		kvm_for_each_memslot(slot, kvm->memslots) {
@@ -791,6 +792,7 @@ int __kvm_set_memory_region(struct kvm *kvm,
 			      (base_gfn >= slot->base_gfn + slot->npages)))
 				goto out;
 		}
+	}
 
 	/* Free page dirty bitmap if unneeded */
 	if (!(new.flags & KVM_MEM_LOG_DIRTY_PAGES))