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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>
This commit is contained in:
Anton Blanchard 2015-10-29 11:43:53 +11:00 committed by Michael Ellerman
parent 31a40e2b05
commit 07e45c120c
1 changed files with 1 additions and 14 deletions
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15
arch/powerpc/kernel/entry_64.S
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@ -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 */