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

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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@shareable.org>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Russell King <linux@arm.linux.org.uk>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: H. J. Lu <hjl.tools@gmail.com>
Link: http://lkml.kernel.org/r/511ACDFB.1050707@zytor.com
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>
This commit is contained in:
H. Peter Anvin 2013-02-12 15:37:02 -08:00
parent 3578baaed4
commit ff52c3b02b
1 changed files with 8 additions and 1 deletions
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9
arch/x86/include/asm/uaccess.h
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@ -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) \
({ \