xen: release all pages within 1-1 p2m mappings

In xen_memory_setup() all reserved regions and gaps are set to an
identity (1-1) p2m mapping.  If an available page has a PFN within one
of these 1-1 mappings it will become inaccessible (as it MFN is lost)
so release them before setting up the mapping.

This can make an additional 256 MiB or more of RAM available
(depending on the size of the reserved regions in the memory map) if
the initial pages overlap with reserved regions.

The 1:1 p2m mappings are also extended to cover partial pages.  This
fixes an issue with (for example) systems with a BIOS that puts the
DMI tables in a reserved region that begins on a non-page boundary.

Signed-off-by: David Vrabel <david.vrabel@citrix.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>

arch/x86/xen/setup.c

@@ -83,25 +83,18 @@ static void __init xen_add_extra_mem(u64 start, u64 size)
 		__set_phys_to_machine(pfn, INVALID_P2M_ENTRY);
 }
 
-static unsigned long __init xen_release_chunk(phys_addr_t start_addr,
-					      phys_addr_t end_addr)
+static unsigned long __init xen_release_chunk(unsigned long start,
+					      unsigned long end)
 {
 	struct xen_memory_reservation reservation = {
 		.address_bits = 0,
 		.extent_order = 0,
 		.domid        = DOMID_SELF
 	};
-	unsigned long start, end;
 	unsigned long len = 0;
 	unsigned long pfn;
 	int ret;
 
-	start = PFN_UP(start_addr);
-	end = PFN_DOWN(end_addr);
-
-	if (end <= start)
-		return 0;
-
 	for(pfn = start; pfn < end; pfn++) {
 		unsigned long mfn = pfn_to_mfn(pfn);
 
@@ -126,72 +119,52 @@ static unsigned long __init xen_release_chunk(phys_addr_t start_addr,
 	return len;
 }
 
-static unsigned long __init xen_return_unused_memory(
-	unsigned long max_pfn, const struct e820entry *map, int nr_map)
+static unsigned long __init xen_set_identity_and_release(
+	const struct e820entry *list, size_t map_size, unsigned long nr_pages)
 {
-	phys_addr_t max_addr = PFN_PHYS(max_pfn);
-	phys_addr_t last_end = ISA_END_ADDRESS;
+	phys_addr_t start = 0;
 	unsigned long released = 0;
-	int i;
-
-	/* Free any unused memory above the low 1Mbyte. */
-	for (i = 0; i < nr_map && last_end < max_addr; i++) {
-		phys_addr_t end = map[i].addr;
-		end = min(max_addr, end);
-
-		if (last_end < end)
-			released += xen_release_chunk(last_end, end);
-		last_end = max(last_end, map[i].addr + map[i].size);
-	}
-
-	if (last_end < max_addr)
-		released += xen_release_chunk(last_end, max_addr);
-
-	printk(KERN_INFO "released %lu pages of unused memory\n", released);
-	return released;
-}
-
-static unsigned long __init xen_set_identity(const struct e820entry *list,
-					     ssize_t map_size)
-{
-	phys_addr_t last = xen_initial_domain() ? 0 : ISA_END_ADDRESS;
-	phys_addr_t start_pci = last;
-	const struct e820entry *entry;
 	unsigned long identity = 0;
+	const struct e820entry *entry;
 	int i;
 
+	/*
+	 * Combine non-RAM regions and gaps until a RAM region (or the
+	 * end of the map) is reached, then set the 1:1 map and
+	 * release the pages (if available) in those non-RAM regions.
+	 *
+	 * The combined non-RAM regions are rounded to a whole number
+	 * of pages so any partial pages are accessible via the 1:1
+	 * mapping.  This is needed for some BIOSes that put (for
+	 * example) the DMI tables in a reserved region that begins on
+	 * a non-page boundary.
+	 */
 	for (i = 0, entry = list; i < map_size; i++, entry++) {
-		phys_addr_t start = entry->addr;
-		phys_addr_t end = start + entry->size;
+		phys_addr_t end = entry->addr + entry->size;
 
-		if (start < last)
-			start = last;
+		if (entry->type == E820_RAM || i == map_size - 1) {
+			unsigned long start_pfn = PFN_DOWN(start);
+			unsigned long end_pfn = PFN_UP(end);
 
-		if (end <= start)
-			continue;
+			if (entry->type == E820_RAM)
+				end_pfn = PFN_UP(entry->addr);
 
-		/* Skip over the 1MB region. */
-		if (last > end)
-			continue;
+			if (start_pfn < end_pfn) {
+				if (start_pfn < nr_pages)
+					released += xen_release_chunk(
+						start_pfn, min(end_pfn, nr_pages));
 
-		if ((entry->type == E820_RAM) || (entry->type == E820_UNUSABLE)) {
-			if (start > start_pci)
 				identity += set_phys_range_identity(
-						PFN_UP(start_pci), PFN_DOWN(start));
-
-			/* Without saving 'last' we would gooble RAM too
-			 * at the end of the loop. */
-			last = end;
-			start_pci = end;
-			continue;
+					start_pfn, end_pfn);
+			}
+			start = end;
 		}
-		start_pci = min(start, start_pci);
-		last = end;
 	}
-	if (last > start_pci)
-		identity += set_phys_range_identity(
-					PFN_UP(start_pci), PFN_DOWN(last));
-	return identity;
+
+	printk(KERN_INFO "Released %lu pages of unused memory\n", released);
+	printk(KERN_INFO "Set %ld page(s) to 1-1 mapping\n", identity);
+
+	return released;
 }
 
 static unsigned long __init xen_get_max_pages(void)
@@ -232,7 +205,6 @@ char * __init xen_memory_setup(void)
 	struct xen_memory_map memmap;
 	unsigned long max_pages;
 	unsigned long extra_pages = 0;
-	unsigned long identity_pages = 0;
 	int i;
 	int op;
 
@@ -265,8 +237,13 @@ char * __init xen_memory_setup(void)
 	if (max_pages > max_pfn)
 		extra_pages += max_pages - max_pfn;
 
-	xen_released_pages = xen_return_unused_memory(max_pfn, map,
-						      memmap.nr_entries);
+	/*
+	 * Set P2M for all non-RAM pages and E820 gaps to be identity
+	 * type PFNs.  Any RAM pages that would be made inaccesible by
+	 * this are first released.
+	 */
+	xen_released_pages = xen_set_identity_and_release(
+		map, memmap.nr_entries, max_pfn);
 	extra_pages += xen_released_pages;
 
 	/*
@@ -312,10 +289,6 @@ char * __init xen_memory_setup(void)
 	 * In domU, the ISA region is normal, usable memory, but we
 	 * reserve ISA memory anyway because too many things poke
 	 * about in there.
-	 *
-	 * In Dom0, the host E820 information can leave gaps in the
-	 * ISA range, which would cause us to release those pages.  To
-	 * avoid this, we unconditionally reserve them here.
 	 */
 	e820_add_region(ISA_START_ADDRESS, ISA_END_ADDRESS - ISA_START_ADDRESS,
 			E820_RESERVED);
@@ -332,12 +305,6 @@ char * __init xen_memory_setup(void)
 
 	sanitize_e820_map(e820.map, ARRAY_SIZE(e820.map), &e820.nr_map);
 
-	/*
-	 * Set P2M for all non-RAM pages and E820 gaps to be identity
-	 * type PFNs.
-	 */
-	identity_pages = xen_set_identity(e820.map, e820.nr_map);
-	printk(KERN_INFO "Set %ld page(s) to 1-1 mapping.\n", identity_pages);
 	return "Xen";
 }