[PATCH] Optimize PDA accesses slightly

Based on a idea by Jeremy Fitzhardinge:

Replace the volatiles and memory clobbers in the PDA access with
telling gcc about access to a proxy PDA structure that doesn't
actually exist. But the dummy accesses give a defined ordering for
read/write accesses.

Also add some memory barriers to the early GS initialization to
make sure no PDA access is moved before it.

Advantage is some .text savings (probably most from better
code for accessing "current"):

   text    data     bss     dec     hex filename
4845647 1223688  615864 6685199  66020f vmlinux
4837780 1223688  615864 6677332  65e354 vmlinux-pda

1.2% smaller code

Cc:  Jeremy Fitzhardinge <jeremy@goop.org>
Signed-off-by: Andi Kleen <ak@suse.de>

1 files changed, 3 insertions, 0 deletions

diff --git a/arch/x86_64/kernel/setup64.c b/arch/x86_64/kernel/setup64.c
index e85cfbb49b63..491361752c70 100644
--- a/arch/x86_64/kernel/setup64.c
+++ b/arch/x86_64/kernel/setup64.c
@@ -121,7 +121,10 @@ void pda_init(int cpu)
 
         /* Setup up data that may be needed in __get_free_pages early */
         asm volatile("movl %0,%%fs ; movl %0,%%gs" :: "r" (0)); 
+        /* Memory clobbers used to order PDA accessed */
+        mb();
         wrmsrl(MSR_GS_BASE, pda);
+        mb();
 
         pda->cpunumber = cpu; 
         pda->irqcount = -1;