[PATCH] fix dubious segment register clear in cpu_init()

[PATCH] fix dubious segment register clear in cpu_init()

[Jeremy Fitzhardinge]

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@goop.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);

Re: [PATCH] fix dubious segment register clear in cpu_init()

[Andi Kleen]

On Friday 04 August 2006 01:49, Jeremy Fitzhardinge wrote:
> 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.

Added thanks

-Andi