xen: release unused free memory

Scan an e820 table and release any memory which lies between e820 entries,
as it won't be used and would just be wasted.  At present this is just to
release any memory beyond the end of the e820 map, but it will also deal
with holes being punched in the map.

Derived from patch by Miroslav Rezanina <mrezanin@redhat.com>
Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>

arch/x86/xen/enlighten.c

@@ -731,7 +731,6 @@ static void set_xen_basic_apic_ops(void)
 
 #endif
 
-
 static void xen_clts(void)
 {
 	struct multicall_space mcs;

arch/x86/xen/setup.c

@@ -20,6 +20,7 @@
 #include <xen/page.h>
 #include <xen/interface/callback.h>
 #include <xen/interface/physdev.h>
+#include <xen/interface/memory.h>
 #include <xen/features.h>
 
 #include "xen-ops.h"
@@ -32,6 +33,56 @@ extern void xen_sysenter_target(void);
 extern void xen_syscall_target(void);
 extern void xen_syscall32_target(void);
 
+static unsigned long __init xen_release_chunk(phys_addr_t start_addr, phys_addr_t end_addr)
+{
+	struct xen_memory_reservation reservation = {
+		.address_bits = 0,
+		.extent_order = 0,
+		.domid        = DOMID_SELF
+	};
+	unsigned long *mfn_list = (unsigned long *)xen_start_info->mfn_list;
+	unsigned long start, end;
+	unsigned long len;
+	unsigned long pfn;
+	int ret;
+
+	start = PFN_UP(start_addr);
+	end = PFN_UP(end_addr);
+
+	if (end <= start)
+		return 0;
+
+	len = end - start;
+
+	set_xen_guest_handle(reservation.extent_start, &mfn_list[start]);
+	reservation.nr_extents = len;
+
+	ret = HYPERVISOR_memory_op(XENMEM_decrease_reservation, &reservation);
+	WARN(ret != (end - start), "Failed to release memory %lx-%lx err=%d\n",
+	     start, end, ret);
+
+	for(pfn = start; pfn < end; pfn++)
+		set_phys_to_machine(pfn, INVALID_P2M_ENTRY);
+
+	return len;
+}
+
+static unsigned long __init xen_return_unused_memory(const struct e820map *e820)
+{
+	unsigned long last_end = 0;
+	unsigned long released = 0;
+	int i;
+
+	for (i = 0; i < e820->nr_map; i++) {
+		released += xen_release_chunk(last_end, e820->map[i].addr);
+		last_end = e820->map[i].addr + e820->map[i].size;
+	}
+
+	released += xen_release_chunk(last_end, PFN_PHYS(xen_start_info->nr_pages));
+
+	printk(KERN_INFO "released %ld pages of unused memory\n", released);
+	return released;
+}
 
 /**
  * machine_specific_memory_setup - Hook for machine specific memory setup.
@@ -67,6 +118,8 @@ char * __init xen_memory_setup(void)
 
 	sanitize_e820_map(e820.map, ARRAY_SIZE(e820.map), &e820.nr_map);
 
+	xen_return_unused_memory(&e820);
+
 	return "Xen";
 }