Index: sys/dev/xen/blkfront/blkfront.c
===================================================================
--- sys/dev/xen/blkfront/blkfront.c
+++ sys/dev/xen/blkfront/blkfront.c
@@ -263,12 +263,19 @@
 		    sizeof(grant_ref_t) * sc->xbd_max_request_indirectpages);
 	}
 
-	if (cm->cm_operation == BLKIF_OP_READ)
+	switch (cm->cm_operation) {
+	case BLKIF_OP_READ:
 		op = BUS_DMASYNC_PREREAD;
-	else if (cm->cm_operation == BLKIF_OP_WRITE)
+		break;
+	case BLKIF_OP_WRITE:
+	case BLKIF_OP_WRITE_BARRIER:
+	case BLKIF_OP_FLUSH_DISKCACHE:
 		op = BUS_DMASYNC_PREWRITE;
-	else
+		break;
+	default:
 		op = 0;
+	}
+
 	bus_dmamap_sync(sc->xbd_io_dmat, cm->cm_map, op);
 
 	gnttab_free_grant_references(cm->cm_gref_head);