[Xen-changelog] [xen-unstable] x86_64: Ensure frame-table compression leaves MAX_ORDER aligned

[Xen patchbot-unstable <patchbot-unstable@xxxxxxxxxxxxxxxxxxx>]

# HG changeset patch
# User Keir Fraser <keir.fraser@xxxxxxxxxx>
# Date 1283332669 -3600
# Node ID 972d90ff31349816cc8c785a8b2defb2f70ed441
# Parent  0eb89b465e46cbc688bcce5f451e125c1650b353
x86_64: Ensure frame-table compression leaves MAX_ORDER aligned
contiguous ranges of page_info structs. This allows page-pointer
arithmetic in places like our buddy allocator.

This restriction was already implicitly guaranteed, but it is good to
make it explicit in the pdx-related initialisation.

Signed-off-by: Keir Fraser <keir.fraser@xxxxxxxxxx>
---
 xen/arch/x86/x86_64/mm.c |   12 +++++++++---
 1 files changed, 9 insertions(+), 3 deletions(-)

diff -r 0eb89b465e46 -r 972d90ff3134 xen/arch/x86/x86_64/mm.c
--- a/xen/arch/x86/x86_64/mm.c  Wed Sep 01 10:17:29 2010 +0100
+++ b/xen/arch/x86/x86_64/mm.c  Wed Sep 01 10:17:49 2010 +0100
@@ -163,9 +163,15 @@ void *do_page_walk(struct vcpu *v, unsig
 
 void __init pfn_pdx_hole_setup(unsigned long mask)
 {
-    unsigned int i, j, bottom_shift, hole_shift;
-
-    for ( hole_shift = bottom_shift = j = 0; ; )
+    unsigned int i, j, bottom_shift = 0, hole_shift = 0;
+
+    /*
+     * We skip the first MAX_ORDER bits, as we never want to compress them.
+     * This guarantees that page-pointer arithmetic remains valid within
+     * contiguous aligned ranges of 2^MAX_ORDER pages. Among others, our
+     * buddy allocator relies on this assumption.
+     */
+    for ( j = MAX_ORDER-1; ; )
     {
         i = find_next_zero_bit(&mask, BITS_PER_LONG, j);
         j = find_next_bit(&mask, BITS_PER_LONG, i);

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