Commit:     f29192320f22f1617d50a9c790862603eeef64e6
Parent:     015c8dd0cb3b380cb4c3930968250c719d1dd303
Author:     Jeremy Fitzhardinge <[EMAIL PROTECTED]>
AuthorDate: Wed Jan 30 13:32:59 2008 +0100
Committer:  Ingo Molnar <[EMAIL PROTECTED]>
CommitDate: Wed Jan 30 13:32:59 2008 +0100

    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 <[EMAIL PROTECTED]>
    Cc: Andi Kleen <[EMAIL PROTECTED]>
    Signed-off-by: Ingo Molnar <[EMAIL PROTECTED]>
    Signed-off-by: Thomas Gleixner <[EMAIL PROTECTED]>
 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 a1eeacd..75a656e 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_RW       (_AC(1, L)<<_PAGE_BIT_RW)
 #define _PAGE_USER     (_AC(1, L)<<_PAGE_BIT_USER)
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