xen: clean up "extra" memory handling some more

Make sure that extra_pages is added for all E820_RAM regions beyond
mem_end - completely excluded regions as well as the remains of partially
included regions.

Also makes sure the extra region is not unnecessarily high, and simplifies
the logic to decide which regions should be added.
Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>

arch/x86/xen/setup.c

@@ -182,24 +182,21 @@ char * __init xen_memory_setup(void)
 	for (i = 0; i < memmap.nr_entries; i++) {
 		unsigned long long end = map[i].addr + map[i].size;
 
-		if (map[i].type == E820_RAM) {
-			if (map[i].addr < mem_end && end > mem_end) {
-				/* Truncate region to max_mem. */
-				u64 delta = end - mem_end;
+		if (map[i].type == E820_RAM && end > mem_end) {
+			/* RAM off the end - may be partially included */
+			u64 delta = min(map[i].size, end - mem_end);
 
 			map[i].size -= delta;
-				extra_pages += PFN_DOWN(delta);
+			end -= delta;
 
-				end = mem_end;
-			}
+			extra_pages += PFN_DOWN(delta);
 		}
 
-		if (end > xen_extra_mem_start)
+		if (map[i].size > 0 && end > xen_extra_mem_start)
 			xen_extra_mem_start = end;
 
-		/* If region is non-RAM or below mem_end, add what remains */
-		if ((map[i].type != E820_RAM || map[i].addr < mem_end) &&
-		    map[i].size > 0)
+		/* Add region if any remains */
+		if (map[i].size > 0)
 			e820_add_region(map[i].addr, map[i].size, map[i].type);
 	}