powerpc/64s: syscall real mode entry use mtmsrd rather than rfid

Have the real mode system call entry handler branch to the kernel
0xc000... address and then use mtmsrd to enable the MMU, rather than use
SRRs and rfid.

Commit 8729c26e ("powerpc/64s/exception: Move real to virt switch
into the common handler") implemented this style of real mode entry for
other interrupt handlers, so this brings system calls into line with
them, which is the main motivcation for the change.

This tends to be slightly faster due to avoiding the mtsprs, and it also
does not clobber the SRR registers, which becomes important in a
subsequent change. The real mode entry points don't tend to be too
important for performance these days, but it is possible for a
hypervisor to run guests in AIL=0 mode for certian reasons.
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210208063326.331502-1-npiggin@gmail.com

arch/powerpc/kernel/entry_64.S

@@ -225,6 +225,12 @@ _ASM_NOKPROBE_SYMBOL(system_call_vectored_emulate)
 	b	system_call_vectored_common
 #endif
 
+	.balign IFETCH_ALIGN_BYTES
+	.globl system_call_common_real
+system_call_common_real:
+	ld	r10,PACAKMSR(r13)	/* get MSR value for kernel */
+	mtmsrd	r10
+
 	.balign IFETCH_ALIGN_BYTES
 	.globl system_call_common
 system_call_common:

arch/powerpc/kernel/exceptions-64s.S

@@ -1973,12 +1973,9 @@ END_FTR_SECTION_IFSET(CPU_FTR_REAL_LE)
 	HMT_MEDIUM
 
 	.if ! \virt
-	__LOAD_HANDLER(r10, system_call_common)
-	mtspr	SPRN_SRR0,r10
-	ld	r10,PACAKMSR(r13)
-	mtspr	SPRN_SRR1,r10
-	RFI_TO_KERNEL
-	b	.	/* prevent speculative execution */
+	__LOAD_HANDLER(r10, system_call_common_real)
+	mtctr	r10
+	bctr
 	.else
 	li	r10,MSR_RI
 	mtmsrd 	r10,1			/* Set RI (EE=0) */