linux-kernel - [PATCH] fix dubious segment register clear in cpu

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Date:	Thu, 03 Aug 2006 16:49:03 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Andrew Morton <akpm@...l.org>, Andi Kleen <ak@...e.de>
CC:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: [PATCH] fix dubious segment register clear in cpu_init()

Fix a very dubious piece of code in
arch/i386/kernel/cpu/common.c:cpu_init().  This clears out %fs and
%gs, but clobbers %eax in the process without telling gcc.  It turns
out that gcc happens to be not using %eax at that point anyway so it
doesn't matter much, but it looks like a bomb waiting to go off.

This does end up saving an instruction, because gcc wants %eax==0 for
the set_debugreg()s below.

Signed-off-by: Jeremy Fitzhardinge <jeremy@...p.org>

diff -r ced3124154dc arch/i386/kernel/cpu/common.c
--- a/arch/i386/kernel/cpu/common.c	Mon Jul 31 18:52:19 2006 -0700
+++ b/arch/i386/kernel/cpu/common.c	Thu Aug 03 16:23:02 2006 -0700
@@ -675,7 +675,7 @@ old_gdt:
 #endif
 
 	/* Clear %fs and %gs. */
-	asm volatile ("xorl %eax, %eax; movl %eax, %fs; movl %eax, %gs");
+	asm volatile ("movl %0, %%fs; movl %0, %%gs" : : "r" (0));
 
 	/* Clear all 6 debug registers: */
 	set_debugreg(0, 0);


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