powerpc: Create context switch helpers save_sprs() and restore_sprs()

Move all our context switch SPR save and restore code into two
helpers. We do a few optimisations:

- Group all mfsprs and all mtsprs. In many cases an mtspr sets a
scoreboarding bit that an mfspr waits on, so the current practise of
mfspr A; mtspr A; mfpsr B; mtspr B is the worst scheduling we can
do.

- SPR writes are slow, so check that the value is changing before
writing it.

A context switch microbenchmark using yield():

http://ozlabs.org/~anton/junkcode/context_switch2.c

./context_switch2 --test=yield 0 0

shows an improvement of almost 10% on POWER8.

Signed-off-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

1 files changed, 0 insertions, 11 deletions

diff --git a/arch/powerpc/include/asm/switch_to.h b/arch/powerpc/include/asm/switch_to.h
index 15cca17cba4b..33a071d24ba8 100644
--- a/arch/powerpc/include/asm/switch_to.h
+++ b/arch/powerpc/include/asm/switch_to.h
@@ -15,17 +15,6 @@ extern struct task_struct *__switch_to(struct task_struct *,
 struct thread_struct;
 extern struct task_struct *_switch(struct thread_struct *prev,
 				   struct thread_struct *next);
-#ifdef CONFIG_PPC_BOOK3S_64
-static inline void save_early_sprs(struct thread_struct *prev)
-{
-	if (cpu_has_feature(CPU_FTR_ARCH_207S))
-		prev->tar = mfspr(SPRN_TAR);
-	if (cpu_has_feature(CPU_FTR_DSCR))
-		prev->dscr = mfspr(SPRN_DSCR);
-}
-#else
-static inline void save_early_sprs(struct thread_struct *prev) {}
-#endif
 
 extern void enable_kernel_fp(void);
 extern void enable_kernel_altivec(void);