Index: sys/arch/x86/x86/bus_space.c
===================================================================
RCS file: /cvsroot/src/sys/arch/x86/x86/bus_space.c,v
retrieving revision 1.38
diff -p -u -r1.38 bus_space.c
--- sys/arch/x86/x86/bus_space.c	27 Jan 2012 18:53:07 -0000	1.38
+++ sys/arch/x86/x86/bus_space.c	18 Mar 2015 13:20:58 -0000
@@ -153,6 +153,8 @@ x86_bus_space_mallocok(void)
 	ioport_malloc_safe = 1;
 }
 
+#include <ddb/ddb.h>
+
 int
 bus_space_map(bus_space_tag_t t, bus_addr_t bpa, bus_size_t size,
 		int flags, bus_space_handle_t *bshp)
@@ -161,6 +163,12 @@ bus_space_map(bus_space_tag_t t, bus_add
 	bus_space_tag_t it;
 	int error;
 
+	if (bpa <= 0xc0000 && 0xc0000 - bpa < size) {
+		printf("%s: mapping vga rom\n", __func__);
+		db_stack_trace_print((db_expr_t)(intptr_t)
+		    __builtin_frame_address(0), true, 65536, "", printf);
+	}
+
 	if ((t->bst_exists & BUS_SPACE_OVERRIDE_MAP) == 0)
 		;	/* skip override */
 	else for (it = t; it != NULL; it = it->bst_super) {