From ff52c3b02b3f73178bfe0c219cd22abdcb0e46c3 Mon Sep 17 00:00:00 2001 From: "H. Peter Anvin" Date: Tue, 12 Feb 2013 15:37:02 -0800 Subject: x86, doc: Clarify the use of asm("%edx") in uaccess.h MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit 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 Cc: Ville Syrjälä Cc: Borislav Petkov Cc: Russell King Cc: Linus Torvalds Cc: H. J. Lu Link: http://lkml.kernel.org/r/511ACDFB.1050707@zytor.com Signed-off-by: H. Peter Anvin --- 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 d710a2555fd6..5ee26875baea 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) \ ({ \ -- cgit v1.2.3