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authorJeremy Fitzhardinge <jeremy@goop.org>2008-01-30 13:32:59 +0100
committerIngo Molnar <mingo@elte.hu>2008-01-30 13:32:59 +0100
commitf29192320f22f1617d50a9c790862603eeef64e6 (patch)
treec496f893ee5d4e8c9636756483411a6ef46c6721 /include
parent015c8dd0cb3b380cb4c3930968250c719d1dd303 (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.h5
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)