[patch 05/10] x86: set PAE PHYSICAL_MASK_SHIFT to 44 bits.

Greg KH
2008-06-23T23:08:41Z

[-- Attachment #1: x86-set-pae-physical_mask_shift-to-44-bits.patch --]
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2.6.25.9-stable review patch. If anyone has any objections, please let
us know.

------------------
From: Jeremy Fitzhardinge <jeremy@goop.org>

commit ad524d46f36bbc32033bb72ba42958f12bf49b06 upstream

When a 64-bit x86 processor runs in 32-bit PAE mode, a pte can
potentially have the same number of physical address bits as the
64-bit host ("Enhanced Legacy PAE Paging"). This means, in theory,
we could have up to 52 bits of physical address in a pte.

The 32-bit kernel uses a 32-bit unsigned long to represent a pfn.
This means that it can only represent physical addresses up to 32+12=44
bits wide. Rather than widening pfns everywhere, just set 2^44 as the
Linux x86_32-PAE architectural limit for physical address size.

This is a bugfix for two cases:
1. running a 32-bit PAE kernel on a machine with
more than 64GB RAM.
2. running a 32-bit PAE Xen guest on a host machine with
more than 64GB RAM

In both cases, a pte could need to have more than 36 bits of physical,
and masking it to 36-bits will cause fairly severe havoc.

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Cc: Jan Beulich <jbeulich@novell.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

---
include/asm-x86/page_32.h | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)

--- a/include/asm-x86/page_32.h
+++ b/include/asm-x86/page_32.h
@@ -14,7 +14,8 @@
#define __PAGE_OFFSET _AC(CONFIG_PAGE_OFFSET, UL)

#ifdef CONFIG_X86_PAE
-#define __PHYSICAL_MASK_SHIFT 36
+/* 44=32+12, the limit we can fit into an unsigned long pfn */
+#define __PHYSICAL_MASK_SHIFT 44
#define __VIRTUAL_MASK_SHIFT 32
#define PAGETABLE_LEVELS 3

--