Index: aac.c
===================================================================
--- aac.c	(revision 232159)
+++ aac.c	(working copy)
@@ -224,6 +224,11 @@
 /* sysctl node */
 static SYSCTL_NODE(_hw, OID_AUTO, aac, CTLFLAG_RD, 0, "AAC driver parameters");
 
+static int aac_allow_64bit = 1;
+TUNABLE_INT("hw.aac.allow_64bit", &aac_allow_64bit);
+SYSCTL_INT(_hw_aac, OID_AUTO, allow_64bit, CTLFLAG_RDTUN, &aac_allow_64bit,
+    0, "Allow 64-bit addressing");
+
 /*
  * Device Interface
  */
@@ -1759,7 +1764,7 @@
 		if (options & AAC_SUPPORTED_NONDASD)
 			sc->flags |= AAC_FLAGS_ENABLE_CAM;
 		if ((options & AAC_SUPPORTED_SGMAP_HOST64) != 0
-		     && (sizeof(bus_addr_t) > 4)) {
+		     && (sizeof(bus_addr_t) > 4) && aac_allow_64bit != 0) {
 			device_printf(sc->aac_dev,
 			    "Enabling 64-bit address support\n");
 			sc->flags |= AAC_FLAGS_SG_64BIT;