powerpc: Don't disable kernel FP/VMX/VSX MSR bits on context switch

Writing the MSR is slow, so we want to avoid it whenever possible.

A subsequent patch will add a debug option that strictly manages the
FP/VMX/VSX unavailable bits. For now just remove it, matching what
we do in other areas of the kernel (eg enable_kernel_altivec()).

A context switch microbenchmark using yield():

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

./context_switch2 --test=yield --fp 0 0

shows an improvement of almost 3% on POWER8.

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

diff --git a/arch/powerpc/kernel/entry_64.S b/arch/powerpc/kernel/entry_64.S
index a94f155db78e83d67fca2e967ba43585f18aeda8..93bb284fddf9415cfa8f5fc0e0ad5bfefe59ebef 100644 (file)
--- a/arch/powerpc/kernel/entry_64.S
+++ b/arch/powerpc/kernel/entry_64.S
@@ -453,26 +453,13 @@ _GLOBAL(_switch)
        SAVE_8GPRS(14, r1)
        SAVE_10GPRS(22, r1)
        mflr    r20             /* Return to switch caller */
-       mfmsr   r22
-       li      r0, MSR_FP
-#ifdef CONFIG_VSX
-BEGIN_FTR_SECTION
-       oris    r0,r0,MSR_VSX@h /* Disable VSX */
-END_FTR_SECTION_IFSET(CPU_FTR_VSX)
-#endif /* CONFIG_VSX */
 #ifdef CONFIG_ALTIVEC
 BEGIN_FTR_SECTION
-       oris    r0,r0,MSR_VEC@h /* Disable altivec */
        mfspr   r24,SPRN_VRSAVE /* save vrsave register value */
        std     r24,THREAD_VRSAVE(r3)
 END_FTR_SECTION_IFSET(CPU_FTR_ALTIVEC)
 #endif /* CONFIG_ALTIVEC */
-       and.    r0,r0,r22
-       beq+    1f
-       andc    r22,r22,r0
-       MTMSRD(r22)
-       isync
-1:     std     r20,_NIP(r1)
+       std     r20,_NIP(r1)
        mfcr    r23
        std     r23,_CCR(r1)
        std     r1,KSP(r3)      /* Set old stack pointer */