linux-kernel - [RFC PATCH] powerpc/6xx: Don't set back MSR

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Date:   Thu, 31 Jan 2019 17:44:54 +0000 (UTC)
From:   Christophe Leroy <christophe.leroy@....fr>
To:     Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>
Cc:     linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org
Subject: [RFC PATCH] powerpc/6xx: Don't set back MSR_RI before reenabling MMU

By delaying the setting of MSR_RI, a 1% improvment is optained on
null_syscall selftest on an mpc8321.

Without this patch:

root@...ippro:~# ./null_syscall
   1134.33 ns     378.11 cycles

With this patch:

root@...ippro:~# ./null_syscall
   1121.85 ns     373.95 cycles

The drawback is that a machine check during that period
would be unrecoverable, but as only main memory is accessed
during that period, it shouldn't be a concern.

Signed-off-by: Christophe Leroy <christophe.leroy@....fr>
---
 arch/powerpc/kernel/head_32.S | 2 --
 1 file changed, 2 deletions(-)

diff --git a/arch/powerpc/kernel/head_32.S b/arch/powerpc/kernel/head_32.S
index 146385b1c2da..ea28a6ab56ec 100644
--- a/arch/powerpc/kernel/head_32.S
+++ b/arch/powerpc/kernel/head_32.S
@@ -282,8 +282,6 @@ __secondary_hold_acknowledge:
 	stw	r1,GPR1(r11);	\
 	stw	r1,0(r11);	\
 	tovirt(r1,r11);			/* set new kernel sp */	\
-	li	r10,MSR_KERNEL & ~(MSR_IR|MSR_DR); /* can take exceptions */ \
-	MTMSRD(r10);			/* (except for mach check in rtas) */ \
 	stw	r0,GPR0(r11);	\
 	lis	r10,STACK_FRAME_REGS_MARKER@ha; /* exception frame marker */ \
 	addi	r10,r10,STACK_FRAME_REGS_MARKER@l; \
-- 
2.13.3