Index: head/sys/dev/hyperv/vmbus/vmbus.c
===================================================================
--- head/sys/dev/hyperv/vmbus/vmbus.c
+++ head/sys/dev/hyperv/vmbus/vmbus.c
@@ -325,7 +325,13 @@
 uint32_t
 vmbus_gpadl_alloc(struct vmbus_softc *sc)
 {
-	return atomic_fetchadd_int(&sc->vmbus_gpadl, 1);
+	uint32_t gpadl;
+
+again:
+	gpadl = atomic_fetchadd_int(&sc->vmbus_gpadl, 1); 
+	if (gpadl == 0)
+		goto again;
+	return (gpadl);
 }
 
 static int