powerpc/book3s: Split the common exception prolog logic into two section.

This patch splits the common exception prolog logic into three parts to
facilitate reuse of existing code in the next patch. This patch also
re-arranges few instructions in such a way that the second part now deals
with saving register values from paca save area to stack frame, and
the third part deals with saving current register values to stack frame.

The second and third part will be reused in the machine check exception
routine in the subsequent patch.

Please note that this patch does not introduce or change existing code
logic. Instead it is just a code movement and instruction re-ordering.

Patch Acked-by Paul. But made some minor modification (explained above) to
address Paul's comment in the later patch(3).
Signed-off-by: Mahesh Salgaonkar <mahesh@linux.vnet.ibm.com>
Acked-by: Paul Mackerras <paulus@samba.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

arch/powerpc/include/asm/exception-64s.h

@@ -301,9 +301,12 @@ do_kvm_##n:								\
 	beq	4f;			/* if from kernel mode		*/ \
 	ACCOUNT_CPU_USER_ENTRY(r9, r10);				   \
 	SAVE_PPR(area, r9, r10);					   \
-4:	std	r2,GPR2(r1);		/* save r2 in stackframe	*/ \
-	SAVE_4GPRS(3, r1);		/* save r3 - r6 in stackframe	*/ \
-	SAVE_2GPRS(7, r1);		/* save r7, r8 in stackframe	*/ \
+4:	EXCEPTION_PROLOG_COMMON_2(area)					   \
+	EXCEPTION_PROLOG_COMMON_3(n)					   \
+	ACCOUNT_STOLEN_TIME
+
+/* Save original regs values from save area to stack frame. */
+#define EXCEPTION_PROLOG_COMMON_2(area)					   \
 	ld	r9,area+EX_R9(r13);	/* move r9, r10 to stackframe	*/ \
 	ld	r10,area+EX_R10(r13);					   \
 	std	r9,GPR9(r1);						   \
@@ -318,11 +321,16 @@ do_kvm_##n:								\
 	ld	r10,area+EX_CFAR(r13);					   \
 	std	r10,ORIG_GPR3(r1);					   \
 	END_FTR_SECTION_NESTED(CPU_FTR_CFAR, CPU_FTR_CFAR, 66);		   \
+	GET_CTR(r10, area);						   \
+	std	r10,_CTR(r1);
+
+#define EXCEPTION_PROLOG_COMMON_3(n)					   \
+	std	r2,GPR2(r1);		/* save r2 in stackframe	*/ \
+	SAVE_4GPRS(3, r1);		/* save r3 - r6 in stackframe   */ \
+	SAVE_2GPRS(7, r1);		/* save r7, r8 in stackframe	*/ \
 	mflr	r9;			/* Get LR, later save to stack	*/ \
 	ld	r2,PACATOC(r13);	/* get kernel TOC into r2	*/ \
 	std	r9,_LINK(r1);						   \
-	GET_CTR(r10, area);						   \
-	std	r10,_CTR(r1);						   \
 	lbz	r10,PACASOFTIRQEN(r13);				   \
 	mfspr	r11,SPRN_XER;		/* save XER in stackframe	*/ \
 	std	r10,SOFTE(r1);						   \
@@ -332,8 +340,7 @@ do_kvm_##n:								\
 	li	r10,0;							   \
 	ld	r11,exception_marker@toc(r2);				   \
 	std	r10,RESULT(r1);		/* clear regs->result		*/ \
-	std	r11,STACK_FRAME_OVERHEAD-16(r1); /* mark the frame	*/ \
-	ACCOUNT_STOLEN_TIME
+	std	r11,STACK_FRAME_OVERHEAD-16(r1); /* mark the frame	*/
 
 /*
  * Exception vectors.