From f29192320f22f1617d50a9c790862603eeef64e6 Mon Sep 17 00:00:00 2001 From: Jeremy Fitzhardinge Date: Wed, 30 Jan 2008 13:32:59 +0100 Subject: x86/pgtable: explain constant sign extension problem 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 Cc: Andi Kleen Signed-off-by: Ingo Molnar Signed-off-by: Thomas Gleixner --- include/asm-x86/pgtable.h | 5 +++++ 1 file changed, 5 insertions(+) (limited to 'include/asm-x86/pgtable.h') diff --git a/include/asm-x86/pgtable.h b/include/asm-x86/pgtable.h index a1eeacdf9d8c..75a656e6b3f8 100644 --- a/include/asm-x86/pgtable.h +++ b/include/asm-x86/pgtable.h @@ -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) -- cgit v1.2.3