diff --git a/sys/dev/xen/bus/xenpv.c b/sys/dev/xen/bus/xenpv.c
index 080c82c73f08..42f2c88a2885 100644
--- a/sys/dev/xen/bus/xenpv.c
+++ b/sys/dev/xen/bus/xenpv.c
@@ -1,200 +1,204 @@
 /*
  * Copyright (c) 2014 Roger Pau Monné <roger.pau@citrix.com>
  * All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
  *    documentation and/or other materials provided with the distribution.
  *
  * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS AS IS'' AND
  * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
  * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
  * ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
  * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
  * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
  * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
  * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
  * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
  * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
  * SUCH DAMAGE.
  */
 
 #include <sys/cdefs.h>
 __FBSDID("$FreeBSD$");
 
 #include <sys/param.h>
 #include <sys/systm.h>
 #include <sys/bus.h>
 #include <sys/kernel.h>
 #include <sys/module.h>
 #include <sys/pcpu.h>
 #include <sys/rman.h>
 #include <sys/smp.h>
 #include <sys/limits.h>
 #include <sys/vmmeter.h>
 
 #include <vm/vm.h>
 #include <vm/vm_page.h>
 #include <vm/vm_param.h>
 #include <vm/vm_phys.h>
 
 #include <xen/xen-os.h>
 #include <xen/gnttab.h>
 
 #include "xenmem_if.h"
 
 /*
  * Allocate unused physical memory above 4GB in order to map memory
  * from foreign domains. We use memory starting at 4GB in order to
  * prevent clashes with MMIO/ACPI regions.
  *
  * 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.
  */
 #ifdef __amd64__
 #define LOW_MEM_LIMIT	0x100000000ul
 #elif defined(__i386__)
 #define LOW_MEM_LIMIT	0x100000ul
 #else
-#error "Unsupported architecture"
+#define LOW_MEM_LIMIT	0
 #endif
 
 static devclass_t xenpv_devclass;
 
 static void
 xenpv_identify(driver_t *driver, device_t parent)
 {
 	if (!xen_domain())
 		return;
 
 	/* Make sure there's only one xenpv device. */
 	if (devclass_get_device(xenpv_devclass, 0))
 		return;
 
 	/*
 	 * The xenpv bus should be the last to attach in order
 	 * to properly detect if an ISA bus has already been added.
 	 */
 	if (BUS_ADD_CHILD(parent, UINT_MAX, "xenpv", 0) == NULL)
 		panic("Unable to attach xenpv bus.");
 }
 
 static int
 xenpv_probe(device_t dev)
 {
 
 	device_set_desc(dev, "Xen PV bus");
 	return (BUS_PROBE_NOWILDCARD);
 }
 
 static int
 xenpv_attach(device_t dev)
 {
 	int error;
 
 	/*
 	 * Let our child drivers identify any child devices that they
 	 * can find.  Once that is done attach any devices that we
 	 * found.
 	 */
 	error = bus_generic_probe(dev);
 	if (error)
 		return (error);
 
 	error = bus_generic_attach(dev);
 
 	return (error);
 }
 
 static struct resource *
 xenpv_alloc_physmem(device_t dev, device_t child, int *res_id, size_t size)
 {
 	struct resource *res;
 	vm_paddr_t phys_addr;
 	int error;
 
 	res = bus_alloc_resource(child, SYS_RES_MEMORY, res_id, LOW_MEM_LIMIT,
 	    ~0, size, RF_ACTIVE);
 	if (res == NULL)
 		return (NULL);
 
 	phys_addr = rman_get_start(res);
 	error = vm_phys_fictitious_reg_range(phys_addr, phys_addr + size,
 	    VM_MEMATTR_XEN);
 	if (error) {
 		bus_release_resource(child, SYS_RES_MEMORY, *res_id, res);
 		return (NULL);
 	}
 
 	return (res);
 }
 
 static int
 xenpv_free_physmem(device_t dev, device_t child, int res_id, struct resource *res)
 {
 	vm_paddr_t phys_addr;
 	size_t size;
 
 	phys_addr = rman_get_start(res);
 	size = rman_get_size(res);
 
 	vm_phys_fictitious_unreg_range(phys_addr, phys_addr + size);
 	return (bus_release_resource(child, SYS_RES_MEMORY, res_id, res));
 }
 
 static device_method_t xenpv_methods[] = {
 	/* Device interface */
 	DEVMETHOD(device_identify,		xenpv_identify),
 	DEVMETHOD(device_probe,			xenpv_probe),
 	DEVMETHOD(device_attach,		xenpv_attach),
 	DEVMETHOD(device_suspend,		bus_generic_suspend),
 	DEVMETHOD(device_resume,		bus_generic_resume),
 
 	/* Bus interface */
 	DEVMETHOD(bus_add_child,		bus_generic_add_child),
 	DEVMETHOD(bus_alloc_resource,		bus_generic_alloc_resource),
 	DEVMETHOD(bus_release_resource,		bus_generic_release_resource),
 	DEVMETHOD(bus_activate_resource,	bus_generic_activate_resource),
 	DEVMETHOD(bus_deactivate_resource,	bus_generic_deactivate_resource),
 
 	/* Interface to allocate memory for foreign mappings */
 	DEVMETHOD(xenmem_alloc,			xenpv_alloc_physmem),
 	DEVMETHOD(xenmem_free,			xenpv_free_physmem),
 
 	DEVMETHOD_END
 };
 
 static driver_t xenpv_driver = {
 	"xenpv",
 	xenpv_methods,
 	0,
 };
 
 DRIVER_MODULE(xenpv, nexus, xenpv_driver, xenpv_devclass, 0, 0);
 
 struct resource *
 xenmem_alloc(device_t dev, int *res_id, size_t size)
 {
 	device_t parent;
 
 	parent = device_get_parent(dev);
 	if (parent == NULL)
 		return (NULL);
 	return (XENMEM_ALLOC(parent, dev, res_id, size));
 }
 
 int
 xenmem_free(device_t dev, int res_id, struct resource *res)
 {
 	device_t parent;
 
 	parent = device_get_parent(dev);
 	if (parent == NULL)
 		return (ENXIO);
 	return (XENMEM_FREE(parent, dev, res_id, res));
 }