powerpc/64s: Optimise MSR handling in exception handling

mtmsrd with L=1 only affects MSR_EE and MSR_RI bits, and we always
know what state those bits are, so the kernel MSR does not need to be
loaded when modifying them.

mtmsrd is often in the critical execution path, so avoiding dependency
on even L1 load is noticable. On a POWER8 this saves about 3 cycles
from the syscall path, and possibly a few from other exception returns
(not measured).

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/kernel/entry_64.S

diff --git a/arch/powerpc/kernel/entry_64.S b/arch/powerpc/kernel/entry_64.S
index 5afd03e..51df82b 100644
--- a/arch/powerpc/kernel/entry_64.S
+++ b/arch/powerpc/kernel/entry_64.S
@@ -139,7 +139,7 @@
 #ifdef CONFIG_PPC_BOOK3E
 	wrteei	1
 #else
-	ld	r11,PACAKMSR(r13)
+	li	r11,MSR_RI
 	ori	r11,r11,MSR_EE
 	mtmsrd	r11,1
 #endif /* CONFIG_PPC_BOOK3E */
@@ -195,7 +195,6 @@
 #ifdef CONFIG_PPC_BOOK3E
 	wrteei	0
 #else
-	ld	r10,PACAKMSR(r13)
 	/*
 	 * For performance reasons we clear RI the same time that we
 	 * clear EE. We only need to clear RI just before we restore r13
@@ -203,8 +202,7 @@
 	 * We have to be careful to restore RI if we branch anywhere from
 	 * here (eg syscall_exit_work).
 	 */
-	li	r9,MSR_RI
-	andc	r11,r10,r9
+	li	r11,0
 	mtmsrd	r11,1
 #endif /* CONFIG_PPC_BOOK3E */
 
@@ -221,13 +219,12 @@
 #endif
 2:	addi    r3,r1,STACK_FRAME_OVERHEAD
 #ifdef CONFIG_PPC_BOOK3S
+	li	r10,MSR_RI
 	mtmsrd	r10,1		/* Restore RI */
 #endif
 	bl	restore_math
 #ifdef CONFIG_PPC_BOOK3S
-	ld	r10,PACAKMSR(r13)
-	li	r9,MSR_RI
-	andc	r11,r10,r9 /* Re-clear RI */
+	li	r11,0
 	mtmsrd	r11,1
 #endif
 	ld	r8,_MSR(r1)
@@ -308,6 +305,7 @@
 	
 syscall_exit_work:
 #ifdef CONFIG_PPC_BOOK3S
+	li	r10,MSR_RI
 	mtmsrd	r10,1		/* Restore RI */
 #endif
 	/* If TIF_RESTOREALL is set, don't scribble on either r3 or ccr.
@@ -354,7 +352,7 @@
 #ifdef CONFIG_PPC_BOOK3E
 	wrteei	1
 #else
-	ld	r10,PACAKMSR(r13)
+	li	r10,MSR_RI
 	ori	r10,r10,MSR_EE
 	mtmsrd	r10,1
 #endif /* CONFIG_PPC_BOOK3E */
@@ -619,7 +617,7 @@
 #ifdef CONFIG_PPC_BOOK3E
 	wrteei	0
 #else
-	ld	r10,PACAKMSR(r13) /* Get kernel MSR without EE */
+	li	r10,MSR_RI
 	mtmsrd	r10,1		  /* Update machine state */
 #endif /* CONFIG_PPC_BOOK3E */
 
@@ -751,7 +749,7 @@
 #ifdef CONFIG_PPC_BOOK3E
 	wrteei	0
 #else
-	ld	r10,PACAKMSR(r13) /* Get kernel MSR without EE */
+	li	r10,MSR_RI
 	mtmsrd	r10,1		  /* Update machine state */
 #endif /* CONFIG_PPC_BOOK3E */
 #endif /* CONFIG_PREEMPT */
@@ -841,8 +839,7 @@
 	 * userspace and we take an exception after restoring r13,
 	 * we end up corrupting the userspace r13 value.
 	 */
-	ld	r4,PACAKMSR(r13) /* Get kernel MSR without EE */
-	andc	r4,r4,r0	 /* r0 contains MSR_RI here */
+	li	r4,0
 	mtmsrd	r4,1
 
 #ifdef CONFIG_PPC_TRANSACTIONAL_MEM