Keep track of which page is the first available to the boot allocator
to avoid scanning the hole "0 ... first_pg" on every allocation, on
platforms that do not have physical memory starting at address 0x00.

This reduces the Xen boot time by roughly 150 seconds on a small
Altix with 6GB of RAM.

Signed-off-by: Jes Sorensen <jes@sgi.com>

diff -r 66609699c9d0 xen/common/page_alloc.c
--- a/xen/common/page_alloc.c	Fri Dec 08 14:29:42 2006 +0100
+++ b/xen/common/page_alloc.c	Mon Dec 11 15:54:10 2006 +0100
@@ -137,6 +137,8 @@ static void map_alloc(unsigned long firs
 }
 
 
+static unsigned long first_pg = -1UL;
+
 static void map_free(unsigned long first_page, unsigned long nr_pages)
 {
     unsigned long start_off, end_off, curr_idx, end_idx;
@@ -152,6 +154,13 @@ static void map_free(unsigned long first
     start_off = first_page & (PAGES_PER_MAPWORD-1);
     end_idx   = (first_page + nr_pages) / PAGES_PER_MAPWORD;
     end_off   = (first_page + nr_pages) & (PAGES_PER_MAPWORD-1);
+
+    /*
+     * Set first_pg to the first entry initialized so the boot
+     * allocator can avoid scanning the hole from 0 for no reason
+     */
+    if (first_page < first_pg)
+        first_pg = first_page;
 
     if ( curr_idx == end_idx )
     {
@@ -258,7 +267,7 @@ unsigned long alloc_boot_pages(unsigned 
 {
     unsigned long pg, i = 0;
 
-    for ( pg = 0; (pg + nr_pfns) < max_page; pg += pfn_align )
+    for ( pg = first_pg; (pg + nr_pfns) < max_page; pg += pfn_align )
     {
         i = alloc_boot_pages_at(nr_pfns, pg);
         if (i != 0)
@@ -301,7 +310,7 @@ void end_boot_allocator(void)
                 INIT_LIST_HEAD(&heap[i][j][k]);
 
     /* Pages that are free now go to the domain sub-allocator. */
-    for ( i = 0; i < max_page; i++ )
+    for ( i = first_pg; i < max_page; i++ )
     {
         curr_free = next_free;
         next_free = !allocated_in_map(i+1);
@@ -482,7 +491,7 @@ void scrub_heap_pages(void)
 
     printk("Scrubbing Free RAM: ");
 
-    for ( pfn = 0; pfn < max_page; pfn++ )
+    for ( pfn = first_pg; pfn < max_page; pfn++ )
     {
         /* Every 100MB, print a progress dot. */
         if ( (pfn % ((100*1024*1024)/PAGE_SIZE)) == 0 )