x86, 64-bit: Set data segments to null after switching to 64-bit mode

This prevents kernel threads from inheriting non-null segment
selectors, and causing optimizations in __switch_to() to be
ineffective.

Signed-off-by: Brian Gerst <brgerst@gmail.com>
Cc: Tim Blechmann <tim@klingt.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Jeremy Fitzhardinge <jeremy@goop.org>
Cc: Jan Beulich <JBeulich@novell.com>
LKML-Reference: <1259165856-3512-1-git-send-email-brgerst@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

diff --git a/arch/x86/kernel/head_64.S b/arch/x86/kernel/head_64.S
index 2640660..17ba9ec 100644 (file)
--- a/arch/x86/kernel/head_64.S
+++ b/arch/x86/kernel/head_64.S
@@ -212,8 +212,8 @@ ENTRY(secondary_startup_64)
         */
        lgdt    early_gdt_descr(%rip)
 
-       /* set up data segments. actually 0 would do too */
-       movl $__KERNEL_DS,%eax
+       /* set up data segments */
+       xorl %eax,%eax
        movl %eax,%ds
        movl %eax,%ss
        movl %eax,%es