diff --git a/src/libxl/libxl_conf.c b/src/libxl/libxl_conf.c
index 07221fc4cba328c655825bf6bddd20c8f859a09c..45729d9eb93837128558fade9dfe04070f2dc746 100644
--- a/src/libxl/libxl_conf.c
+++ b/src/libxl/libxl_conf.c
@@ -547,9 +547,13 @@ libxlMakeDisk(virDomainDiskDefPtr l_disk, libxl_device_disk *x_disk)
             return -1;
         }
     } else {
-        /* No driverName - default to raw/tap?? */
+        /*
+         * If driverName is not specified, default to raw as per
+         * xl-disk-configuration.txt in the xen documentation and let
+         * libxl pick a suitable backend.
+         */
         x_disk->format = LIBXL_DISK_FORMAT_RAW;
-        x_disk->backend = LIBXL_DISK_BACKEND_TAP;
+        x_disk->backend = LIBXL_DISK_BACKEND_UNKNOWN;
     }
 
     /* XXX is this right? */