powerpc: Better scheduling of CR save code in system call path

At the moment system call entry looks like:

crclr	so
...
mfcr	r9
...
std	r9,_CCR(r1)

commit bd19c8994a ([POWERPC] system call micro optimisation) put
some space between the crclr and mfcr in order to avoid a stall.

There is still a stall seen between the mfcr and std. We can avoid
the crclr by doing it in a GPR with rlwinm which gives us more room
to better schedule the sequence.

Signed-off-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

arch/powerpc/kernel/entry_64.S

@@ -63,15 +63,9 @@ system_call_common:
 	std	r0,GPR0(r1)
 	std	r10,GPR1(r1)
 	ACCOUNT_CPU_USER_ENTRY(r10, r11)
-	/*
-	 * This "crclr so" clears CR0.SO, which is the error indication on
-	 * return from this system call.  There must be no cmp instruction
-	 * between it and the "mfcr r9" below, otherwise if XER.SO is set,
-	 * CR0.SO will get set, causing all system calls to appear to fail.
-	 */
-	crclr	so
 	std	r2,GPR2(r1)
 	std	r3,GPR3(r1)
+	mfcr	r2
 	std	r4,GPR4(r1)
 	std	r5,GPR5(r1)
 	std	r6,GPR6(r1)
@@ -85,13 +79,17 @@ system_call_common:
 	std	r11,_XER(r1)
 	std	r11,_CTR(r1)
 	std	r9,GPR13(r1)
-	mfcr	r9
 	mflr	r10
+	/*
+	 * This clears CR0.SO (bit 28), which is the error indication on
+	 * return from this system call.
+	 */
+	rldimi	r2,r11,28,(63-28)
 	li	r11,0xc01
-	std	r9,_CCR(r1)
 	std	r10,_LINK(r1)
 	std	r11,_TRAP(r1)
 	std	r3,ORIG_GPR3(r1)
+	std	r2,_CCR(r1)
 	ld	r2,PACATOC(r13)
 	addi	r9,r1,STACK_FRAME_OVERHEAD
 	ld	r11,exception_marker@toc(r2)