linux-kernel - Re: [PATCH UPDATED] x86: set PAE PHYSICAL_MASK

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Date:	Fri, 06 Jun 2008 10:58:05 +0100
From:	"Jan Beulich" <jbeulich@...ell.com>
To:	"Jeremy Fitzhardinge" <jeremy@...p.org>
Cc:	"Ingo Molnar" <mingo@...e.hu>, "Stable Kernel" <stable@...nel.org>,
	<x86@...nel.org>,
	"Linux Kernel Mailing List" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH UPDATED] x86: set PAE PHYSICAL_MASK_SHIFT to 44
	bits.

Ah, yes!

Acked-By: Jan Beulich <jbeulich@...ell.com>

>>> Jeremy Fitzhardinge <jeremy@...p.org> 06.06.08 11:21 >>>
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.

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@...rix.com>
Cc: Jan Beulich <jbeulich@...ell.com>
---
 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
@@ -22,7 +22,8 @@
 
 
 #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
 



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