Index: sys/dev/xen/bus/xenpv.c
===================================================================
--- sys/dev/xen/bus/xenpv.c
+++ sys/dev/xen/bus/xenpv.c
@@ -55,13 +55,18 @@
  *
  * Since this is not possible on i386 just use any available memory
  * chunk above 1MB and hope we don't clash with anything else.
+ *
+ * Other architectures better document MMIO regions and drivers more
+ * reliably reserve them.  As such, allow using any unpopulated memory
+ * region.  The zero-page though is reserved and triggers special
+ * behavior by nexus_alloc_resource().
  */
 #ifdef __amd64__
 #define LOW_MEM_LIMIT	0x100000000ul
 #elif defined(__i386__)
 #define LOW_MEM_LIMIT	0x100000ul
 #else
-#error "Unsupported architecture"
+#define LOW_MEM_LIMIT	PAGE_SIZE
 #endif
 
 static devclass_t xenpv_devclass;