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x86/pgtable: explain constant sign extension problem

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When the _PAGE_FOO constants are defined as (1ul << _PAGE_BIT_FOO), they
become unsigned longs.  In 32-bit PAE mode, these end up being
implicitly cast to 64-bit types when used to manipulate a pte, and
because they're unsigned the top 32-bits are 0, destroying the upper
bits of the pte.

When _PAGE_FOO constants are given a signed integer type, the cast to
64-bits will sign-extend so that the upper bits are all ones,
preserving the upper pte bits in manipulations.

Explain this in a prominent place.

Signed-off-by: Jeremy Fitzhardinge <jeremy@xensource.com>
Cc: Andi Kleen <ak@suse.de>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
This commit is contained in:
Jeremy Fitzhardinge 2008-01-30 13:32:59 +01:00 committed by Ingo Molnar
parent 015c8dd0cb
commit f29192320f
1 changed files with 5 additions and 0 deletions
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5
include/asm-x86/pgtable.h
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@ -19,6 +19,11 @@
#define _PAGE_BIT_UNUSED3 11
#define _PAGE_BIT_NX 63 /* No execute: only valid after cpuid check */
/*
* Note: we use _AC(1, L) instead of _AC(1, UL) so that we get a
* sign-extended value on 32-bit with all 1's in the upper word,
* which preserves the upper pte values on 64-bit ptes:
*/
#define _PAGE_PRESENT (_AC(1, L)<<_PAGE_BIT_PRESENT)
#define _PAGE_RW (_AC(1, L)<<_PAGE_BIT_RW)
#define _PAGE_USER (_AC(1, L)<<_PAGE_BIT_USER)