[PATCH] i386: Convert i386 PDA code to use %fs

Convert the PDA code to use %fs rather than %gs as the segment for
per-processor data.  This is because some processors show a small but
measurable performance gain for reloading a NULL segment selector (as %fs
generally is in user-space) versus a non-NULL one (as %gs generally is).

On modern processors the difference is very small, perhaps undetectable.
Some old AMD "K6 3D+" processors are noticably slower when %fs is used
rather than %gs; I have no idea why this might be, but I think they're
sufficiently rare that it doesn't matter much.

This patch also fixes the math emulator, which had not been adjusted to
match the changed struct pt_regs.

[frederik.deweerdt@gmail.com: fixit with gdb]
[mingo@elte.hu: Fix KVM too]

Signed-off-by: Jeremy Fitzhardinge <jeremy@xensource.com>
Signed-off-by: Andi Kleen <ak@suse.de>
Cc: Ian Campbell <Ian.Campbell@XenSource.com>
Acked-by: Ingo Molnar <mingo@elte.hu>
Acked-by: Zachary Amsden <zach@vmware.com>
Cc: Eric Dumazet <dada1@cosmosbay.com>
Signed-off-by: Frederik Deweerdt <frederik.deweerdt@gmail.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>

1 files changed, 3 insertions, 3 deletions

diff --git a/arch/i386/kernel/head.S b/arch/i386/kernel/head.S
index cb9abdfced9b..15336c8b5960 100644
--- a/arch/i386/kernel/head.S
+++ b/arch/i386/kernel/head.S
@@ -319,12 +319,12 @@ is386:	movl $2,%ecx		# set MP
         movl %eax,%ds
         movl %eax,%es
 
-        xorl %eax,%eax                  # Clear FS and LDT
-        movl %eax,%fs
+        xorl %eax,%eax                  # Clear GS and LDT
+        movl %eax,%gs
         lldt %ax
 
         movl $(__KERNEL_PDA),%eax
-        mov  %eax,%gs
+        mov  %eax,%fs
 
         cld                     # gcc2 wants the direction flag cleared at all times
         pushl $0                # fake return address for unwinder