powerpc: Remove use of a second scratch SPRG in STAB code

The STAB code used on Power3 and RS/64 uses a second scratch SPRG to
save a GPR in order to decide whether to go to do_stab_bolted_* or
to handle a normal data access exception.

This prevents our scheme of freeing SPRG3 which is user visible for
user uses since we cannot use SPRG0 which, on RS/64, seems to be
read-only for supervisor mode (like POWER4).

This reworks the STAB exception entry to use the PACA as temporary
storage instead.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

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

@@ -66,8 +66,7 @@
 	std	r9,area+EX_R13(r13);					\
 	mfcr	r9
 
-#define EXCEPTION_PROLOG_PSERIES(area, label)				\
-	EXCEPTION_PROLOG_1(area);					\
+#define EXCEPTION_PROLOG_PSERIES_1(label)				\
 	ld	r12,PACAKBASE(r13);	/* get high part of &label */	\
 	ld	r10,PACAKMSR(r13);	/* get MSR value for kernel */	\
 	mfspr	r11,SPRN_SRR0;		/* save SRR0 */			\
@@ -78,6 +77,10 @@
 	rfid;								\
 	b	.	/* prevent speculative execution */
 
+#define EXCEPTION_PROLOG_PSERIES(area, label)				\
+	EXCEPTION_PROLOG_1(area);					\
+	EXCEPTION_PROLOG_PSERIES_1(label);
+
 /*
  * The common exception prolog is used for all except a few exceptions
  * such as a segment miss on a kernel address.  We have to be prepared

arch/powerpc/include/asm/reg.h

@@ -654,7 +654,7 @@
  * 64-bit server:
  *	- SPRG0 unused (reserved for HV on Power4)
  *	- SPRG1 scratch for exception vectors
- *	- SPRG2 scratch for exception vectors
+ *	- SPRG2 unused
  *
  * All 32-bit:
  *	- SPRG3 current thread_info pointer
@@ -707,7 +707,6 @@
 
 #ifdef CONFIG_PPC_BOOK3S_64
 #define SPRN_SPRG_SCRATCH0	SPRN_SPRG1
-#define SPRN_SPRG_SCRATCH1	SPRN_SPRG2
 #endif
 
 #ifdef CONFIG_PPC_BOOK3S_32

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

@@ -50,18 +50,28 @@ data_access_pSeries:
 	HMT_MEDIUM
 	mtspr	SPRN_SPRG_SCRATCH0,r13
 BEGIN_FTR_SECTION
-	mtspr	SPRN_SPRG_SCRATCH1,r12
-	mfspr	r13,SPRN_DAR
-	mfspr	r12,SPRN_DSISR
-	srdi	r13,r13,60
-	rlwimi	r13,r12,16,0x20
-	mfcr	r12
-	cmpwi	r13,0x2c
+	mfspr	r13,SPRN_SPRG_PACA
+	std	r9,PACA_EXSLB+EX_R9(r13)
+	std	r10,PACA_EXSLB+EX_R10(r13)
+	mfspr	r10,SPRN_DAR
+	mfspr	r9,SPRN_DSISR
+	srdi	r10,r10,60
+	rlwimi	r10,r9,16,0x20
+	mfcr	r9
+	cmpwi	r10,0x2c
 	beq	do_stab_bolted_pSeries
-	mtcrf	0x80,r12
-	mfspr	r12,SPRN_SPRG_SCRATCH1
-END_FTR_SECTION_IFCLR(CPU_FTR_SLB)
+	ld	r10,PACA_EXSLB+EX_R10(r13)
+	std	r11,PACA_EXGEN+EX_R11(r13)
+	ld	r11,PACA_EXSLB+EX_R9(r13)
+	std	r12,PACA_EXGEN+EX_R12(r13)
+	mfspr	r12,SPRN_SPRG_SCRATCH0
+	std	r10,PACA_EXGEN+EX_R10(r13)
+	std	r11,PACA_EXGEN+EX_R9(r13)
+	std	r12,PACA_EXGEN+EX_R13(r13)
+	EXCEPTION_PROLOG_PSERIES_1(data_access_common)
+FTR_SECTION_ELSE
 	EXCEPTION_PROLOG_PSERIES(PACA_EXGEN, data_access_common)
+ALT_FTR_SECTION_END_IFCLR(CPU_FTR_SLB)
 
 	. = 0x380
 	.globl data_access_slb_pSeries
@@ -224,9 +234,11 @@ masked_interrupt:
 
 	.align	7
 do_stab_bolted_pSeries:
-	mtcrf	0x80,r12
-	mfspr	r12,SPRN_SPRG_SCRATCH1
-	EXCEPTION_PROLOG_PSERIES(PACA_EXSLB, .do_stab_bolted)
+	std	r11,PACA_EXSLB+EX_R11(r13)
+	std	r12,PACA_EXSLB+EX_R12(r13)
+	mfspr	r10,SPRN_SPRG_SCRATCH0
+	std	r10,PACA_EXSLB+EX_R13(r13)
+	EXCEPTION_PROLOG_PSERIES_1(.do_stab_bolted)
 
 #ifdef CONFIG_PPC_PSERIES
 /*

arch/powerpc/platforms/iseries/exception.S

@@ -128,25 +128,36 @@ iSeries_secondary_smp_loop:
 data_access_iSeries:
 	mtspr	SPRN_SPRG_SCRATCH0,r13
 BEGIN_FTR_SECTION
-	mtspr	SPRN_SPRG_SCRATCH1,r12
-	mfspr	r13,SPRN_DAR
-	mfspr	r12,SPRN_DSISR
-	srdi	r13,r13,60
-	rlwimi	r13,r12,16,0x20
-	mfcr	r12
-	cmpwi	r13,0x2c
+	mfspr	r13,SPRN_SPRG_PACA
+	std	r9,PACA_EXSLB+EX_R9(r13)
+	std	r10,PACA_EXSLB+EX_R10(r13)
+	mfspr	r10,SPRN_DAR
+	mfspr	r9,SPRN_DSISR
+	srdi	r10,r10,60
+	rlwimi	r10,r9,16,0x20
+	mfcr	r9
+	cmpwi	r10,0x2c
 	beq	.do_stab_bolted_iSeries
-	mtcrf	0x80,r12
-	mfspr	r12,SPRN_SPRG_SCRATCH1
-END_FTR_SECTION_IFCLR(CPU_FTR_SLB)
+	ld	r10,PACA_EXSLB+EX_R10(r13)
+	std	r11,PACA_EXGEN+EX_R11(r13)
+	ld	r11,PACA_EXSLB+EX_R9(r13)
+	std	r12,PACA_EXGEN+EX_R12(r13)
+	mfspr	r12,SPRN_SPRG_SCRATCH0
+	std	r10,PACA_EXGEN+EX_R10(r13)
+	std	r11,PACA_EXGEN+EX_R9(r13)
+	std	r12,PACA_EXGEN+EX_R13(r13)
+	EXCEPTION_PROLOG_ISERIES_1
+FTR_SECTION_ELSE
 	EXCEPTION_PROLOG_1(PACA_EXGEN)
 	EXCEPTION_PROLOG_ISERIES_1
+ALT_FTR_SECTION_END_IFCLR(CPU_FTR_SLB)
 	b	data_access_common
 
 .do_stab_bolted_iSeries:
-	mtcrf	0x80,r12
-	mfspr	r12,SPRN_SPRG_SCRATCH1
-	EXCEPTION_PROLOG_1(PACA_EXSLB)
+	std	r11,PACA_EXSLB+EX_R11(r13)
+	std	r12,PACA_EXSLB+EX_R12(r13)
+	mfspr	r10,SPRN_SPRG_SCRATCH0
+	std	r10,PACA_EXSLB+EX_R13(r13)
 	EXCEPTION_PROLOG_ISERIES_1
 	b	.do_stab_bolted