powerpc/32: Reorder instructions to avoid using CTR in syscall entry

Now that we are using rfi instead of mtmsr to reactivate MMU, it is
possible to reorder instructions and avoid the need to use CTR for
stashing SRR0.

null_syscall on 8xx is reduced by 3 cycles (283 => 280 cycles).

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/8fa13a59f73647e058c95fc7e1c7a98f316bd20a.1612796617.git.christophe.leroy@csgroup.eu

arch/powerpc/kernel/head_32.h

@@ -116,30 +116,28 @@
 .endm
 
 .macro SYSCALL_ENTRY trapno
-	mfspr	r12,SPRN_SPRG_THREAD
 	mfspr	r9, SPRN_SRR1
-	mfspr	r11, SPRN_SRR0
-	mtctr	r11
+	mfspr	r10, SPRN_SRR0
 	andi.	r11, r9, MSR_PR
+	beq-	99f
+	LOAD_REG_IMMEDIATE(r11, MSR_KERNEL)		/* can take exceptions */
+	lis	r12, 1f@h
+	ori	r12, r12, 1f@l
+	mtspr	SPRN_SRR1, r11
+	mtspr	SPRN_SRR0, r12
+	mfspr	r12,SPRN_SPRG_THREAD
 	mr	r11, r1
 	lwz	r1,TASK_STACK-THREAD(r12)
-	beq-	99f
+	tovirt(r12, r12)
 	addi	r1, r1, THREAD_SIZE - INT_FRAME_SIZE
-	LOAD_REG_IMMEDIATE(r10, MSR_KERNEL)		/* can take exceptions */
-	mtspr	SPRN_SRR1, r10
-	lis	r10, 1f@h
-	ori	r10, r10, 1f@l
-	mtspr	SPRN_SRR0, r10
 	rfi
 1:
-	tovirt(r12, r12)
 	stw	r11,GPR1(r1)
 	stw	r11,0(r1)
 	mr	r11, r1
+	stw	r10,_NIP(r11)
 	mflr	r10
 	stw	r10, _LINK(r11)
-	mfctr	r10
-	stw	r10,_NIP(r11)
 	mfcr	r10
 	rlwinm	r10,r10,0,4,2	/* Clear SO bit in CR */
 	stw	r10,_CCR(r11)		/* save registers */