[PATCH for review] [115/145] x86_64: fix dubious segment register clear in cpu_init()

[Andi Kleen]

r

From: Jeremy Fitzhardinge <jeremy@xxxxxxxx>

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@xxxxxxxx>
Signed-off-by: Andi Kleen <ak@xxxxxxx>

---
 arch/i386/kernel/cpu/common.c |    2 +-
 1 files changed, 1 insertion(+), 1 deletion(-)

Index: linux/arch/i386/kernel/cpu/common.c
===================================================================
--- linux.orig/arch/i386/kernel/cpu/common.c
+++ linux/arch/i386/kernel/cpu/common.c
@@ -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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