diff options
| author | Jeremy Fitzhardinge <jeremy@goop.org> | 2008-01-30 13:32:59 +0100 |
|---|---|---|
| committer | Ingo Molnar <mingo@elte.hu> | 2008-01-30 13:32:59 +0100 |
| commit | f29192320f22f1617d50a9c790862603eeef64e6 (patch) | |
| tree | c496f893ee5d4e8c9636756483411a6ef46c6721 /include | |
| parent | 015c8dd0cb3b380cb4c3930968250c719d1dd303 (diff) | |
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 <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>
Diffstat (limited to 'include')
| -rw-r--r-- | include/asm-x86/pgtable.h | 5 |
1 files changed, 5 insertions, 0 deletions
diff --git a/include/asm-x86/pgtable.h b/include/asm-x86/pgtable.h index a1eeacdf9d8..75a656e6b3f 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) |