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>

1 files changed, 5 insertions, 2 deletions

diff --git a/arch/powerpc/include/asm/exception-64s.h b/arch/powerpc/include/asm/exception-64s.h
index 773e380b5fe8..a98653b26231 100644
--- a/arch/powerpc/include/asm/exception-64s.h
+++ b/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