[tip:x86/mm] x86, doc: Clarify the use of asm("%edx") in uaccess.h

From: tip-bot for H. Peter Anvin
Date: Tue Feb 12 2013 - 19:03:37 EST

Commit-ID: ff52c3b02b3f73178bfe0c219cd22abdcb0e46c3
Gitweb: http://git.kernel.org/tip/ff52c3b02b3f73178bfe0c219cd22abdcb0e46c3
Author: H. Peter Anvin <hpa@xxxxxxxxxxxxxxx>
AuthorDate: Tue, 12 Feb 2013 15:37:02 -0800
Committer: H. Peter Anvin <hpa@xxxxxxxxxxxxxxx>
CommitDate: Tue, 12 Feb 2013 15:37:02 -0800

x86, doc: Clarify the use of asm("%edx") in uaccess.h

Put in a comment that explains that the use of asm("%edx") in
uaccess.h doesn't actually necessarily mean %edx alone.

Cc: Jamie Lokier <jamie@xxxxxxxxxxxxx>
Cc: Ville SyrjÃlà <ville.syrjala@xxxxxxxxxxxxxxx>
Cc: Borislav Petkov <bp@xxxxxxxxx>
Cc: Russell King <linux@xxxxxxxxxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: H. J. Lu <hjl.tools@xxxxxxxxx>
Link: http://lkml.kernel.org/r/511ACDFB.1050707@xxxxxxxxx
Signed-off-by: H. Peter Anvin <hpa@xxxxxxxxxxxxxxx>
arch/x86/include/asm/uaccess.h | 9 ++++++++-
1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/arch/x86/include/asm/uaccess.h b/arch/x86/include/asm/uaccess.h
index d710a25..5ee2687 100644
--- a/arch/x86/include/asm/uaccess.h
+++ b/arch/x86/include/asm/uaccess.h
@@ -148,9 +148,16 @@ __typeof__(__builtin_choose_expr(sizeof(x) > sizeof(0UL), 0ULL, 0UL))
* Returns zero on success, or -EFAULT on error.
* On error, the variable @x is set to zero.
- *
+ */
* Careful: we have to cast the result to the type of the pointer
* for sign reasons.
+ *
+ * The use of %edx as the register specifier is a bit of a
+ * simplification, as gcc only cares about it as the starting point
+ * and not size: for a 64-bit value it will use %ecx:%edx on 32 bits
+ * (%ecx being the next register in gcc's x86 register sequence), and
+ * %rdx on 64 bits.
#define get_user(x, ptr) \
({ \
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