powerpc/6xx: Don't use SPRN_SPRG2 for storing stack pointer while in RTAS

When calling RTAS, the stack pointer is stored in SPRN_SPRG2
in order to be able to restore it in case of machine check in RTAS.

As machine check is not a perfomance critical path, this patch
frees SPRN_SPRG2 by using a field in thread struct instead.
Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/include/asm/processor.h

@@ -250,6 +250,9 @@ struct thread_struct {
 #ifdef CONFIG_PPC32
 	void		*pgdir;		/* root of page-table tree */
 	unsigned long	ksp_limit;	/* if ksp <= ksp_limit stack overflow */
+#ifdef CONFIG_PPC_RTAS
+	unsigned long	rtas_sp;	/* stack pointer for when in RTAS */
+#endif
 #endif
 	/* Debug Registers */
 	struct debug_reg debug;

arch/powerpc/include/asm/reg.h

@@ -1167,7 +1167,6 @@
 #ifdef CONFIG_PPC_BOOK3S_32
 #define SPRN_SPRG_SCRATCH0	SPRN_SPRG0
 #define SPRN_SPRG_SCRATCH1	SPRN_SPRG1
-#define SPRN_SPRG_RTAS		SPRN_SPRG2
 #define SPRN_SPRG_603_LRU	SPRN_SPRG4
 #endif
 

arch/powerpc/kernel/asm-offsets.c

@@ -93,6 +93,9 @@ int main(void)
 	OFFSET(THREAD_INFO, task_struct, stack);
 	DEFINE(THREAD_INFO_GAP, _ALIGN_UP(sizeof(struct thread_info), 16));
 	OFFSET(KSP_LIMIT, thread_struct, ksp_limit);
+#ifdef CONFIG_PPC_RTAS
+	OFFSET(RTAS_SP, thread_struct, rtas_sp);
+#endif
 #endif /* CONFIG_PPC64 */
 
 #ifdef CONFIG_LIVEPATCH

arch/powerpc/kernel/entry_32.S

@@ -1332,7 +1332,7 @@ _GLOBAL(enter_rtas)
 	MTMSRD(r0)		/* don't get trashed */
 	li	r9,MSR_KERNEL & ~(MSR_IR|MSR_DR)
 	mtlr	r6
-	mtspr	SPRN_SPRG_RTAS,r7
+	stw	r7, THREAD + RTAS_SP(r2)
 	mtspr	SPRN_SRR0,r8
 	mtspr	SPRN_SRR1,r9
 	RFI
@@ -1341,7 +1341,8 @@ _GLOBAL(enter_rtas)
 	lwz	r9,8(r9)	/* original msr value */
 	addi	r1,r1,INT_FRAME_SIZE
 	li	r0,0
-	mtspr	SPRN_SPRG_RTAS,r0
+	tophys(r7, r2)
+	stw	r0, THREAD + RTAS_SP(r7)
 	mtspr	SPRN_SRR0,r8
 	mtspr	SPRN_SRR1,r9
 	RFI			/* return to caller */

arch/powerpc/kernel/head_32.S

@@ -352,9 +352,8 @@ i##n:								\
  * registers that might have bad values includes all the GPRs
  * and all the BATs.  We indicate that we are in RTAS by putting
  * a non-zero value, the address of the exception frame to use,
- * in SPRG2.  The machine check handler checks SPRG2 and uses its
- * value if it is non-zero.  If we ever needed to free up SPRG2,
- * we could use a field in the thread_info or thread_struct instead.
+ * in thread.rtas_sp.  The machine check handler checks thread.rtas_sp
+ * and uses its value if it is non-zero.
  * (Other exception handlers assume that r1 is a valid kernel stack
  * pointer when we take an exception from supervisor mode.)
  *	-- paulus.
@@ -365,16 +364,15 @@ i##n:								\
 	mtspr	SPRN_SPRG_SCRATCH1,r11
 	mfcr	r10
 #ifdef CONFIG_PPC_CHRP
-	mfspr	r11,SPRN_SPRG_RTAS
-	cmpwi	0,r11,0
-	bne	7f
+	mfspr	r11, SPRN_SPRG_THREAD
+	lwz	r11, RTAS_SP(r11)
+	cmpwi	cr1, r11, 0
+	bne	cr1, 7f
 #endif /* CONFIG_PPC_CHRP */
 	EXCEPTION_PROLOG_1
 7:	EXCEPTION_PROLOG_2
 	addi	r3,r1,STACK_FRAME_OVERHEAD
 #ifdef CONFIG_PPC_CHRP
-	mfspr	r4,SPRN_SPRG_RTAS
-	cmpwi	cr1,r4,0
 	bne	cr1,1f
 #endif
 	EXC_XFER_STD(0x200, machine_check_exception)
@@ -865,8 +863,10 @@ __secondary_start:
 	tophys(r4,r2)
 	addi	r4,r4,THREAD	/* phys address of our thread_struct */
 	mtspr	SPRN_SPRG_THREAD,r4
+#ifdef CONFIG_PPC_RTAS
 	li	r3,0
-	mtspr	SPRN_SPRG_RTAS,r3	/* 0 => not in RTAS */
+	stw	r3, RTAS_SP(r4)		/* 0 => not in RTAS */
+#endif
 
 	/* enable MMU and jump to start_secondary */
 	li	r4,MSR_KERNEL
@@ -950,8 +950,10 @@ start_here:
 	tophys(r4,r2)
 	addi	r4,r4,THREAD	/* init task's THREAD */
 	mtspr	SPRN_SPRG_THREAD,r4
+#ifdef CONFIG_PPC_RTAS
 	li	r3,0
-	mtspr	SPRN_SPRG_RTAS,r3	/* 0 => not in RTAS */
+	stw	r3, RTAS_SP(r4)		/* 0 => not in RTAS */
+#endif
 
 	/* stack */
 	lis	r1,init_thread_union@ha