powerpc/32s: Use SPRN_SPRG_SCRATCH2 in DSI prolog

Use SPRN_SPRG_SCRATCH2 as an alternative scratch register in
the early part of DSI prolog in order to avoid clobbering
SPRN_SPRG_SCRATCH0/1 used by other prologs.

The 603 doesn't like a jump from DataLoadTLBMiss to the 10 nops
that are now in the beginning of DSI exception as a result of
the feature section. To workaround this, add a jump as alternative.
It also avoids fetching 10 nops for nothing.
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/f9f8df2a2be93568768ef1ac793639f7914cf103.1606285014.git.christophe.leroy@csgroup.eu

arch/powerpc/include/asm/reg.h

@@ -1204,6 +1204,7 @@
 #ifdef CONFIG_PPC_BOOK3S_32
 #define SPRN_SPRG_SCRATCH0	SPRN_SPRG0
 #define SPRN_SPRG_SCRATCH1	SPRN_SPRG1
+#define SPRN_SPRG_SCRATCH2	SPRN_SPRG2
 #define SPRN_SPRG_603_LRU	SPRN_SPRG4
 #endif
 

arch/powerpc/kernel/head_book3s_32.S

@@ -288,9 +288,9 @@ MachineCheck:
 	DO_KVM  0x300
 DataAccess:
 #ifdef CONFIG_VMAP_STACK
-	mtspr	SPRN_SPRG_SCRATCH0,r10
-	mfspr	r10, SPRN_SPRG_THREAD
 BEGIN_MMU_FTR_SECTION
+	mtspr	SPRN_SPRG_SCRATCH2,r10
+	mfspr	r10, SPRN_SPRG_THREAD
 	stw	r11, THR11(r10)
 	mfspr	r10, SPRN_DSISR
 	mfcr	r11
@@ -304,19 +304,11 @@ BEGIN_MMU_FTR_SECTION
 .Lhash_page_dsi_cont:
 	mtcr	r11
 	lwz	r11, THR11(r10)
-END_MMU_FTR_SECTION_IFSET(MMU_FTR_HPTE_TABLE)
-	mtspr	SPRN_SPRG_SCRATCH1,r11
-	mfspr	r11, SPRN_DAR
-	stw	r11, DAR(r10)
-	mfspr	r11, SPRN_DSISR
-	stw	r11, DSISR(r10)
-	mfspr	r11, SPRN_SRR0
-	stw	r11, SRR0(r10)
-	mfspr	r11, SPRN_SRR1		/* check whether user or kernel */
-	stw	r11, SRR1(r10)
-	mfcr	r10
-	andi.	r11, r11, MSR_PR
-
+	mfspr	r10, SPRN_SPRG_SCRATCH2
+MMU_FTR_SECTION_ELSE
+	b	1f
+ALT_MMU_FTR_SECTION_END_IFSET(MMU_FTR_HPTE_TABLE)
+1:	EXCEPTION_PROLOG_0 handle_dar_dsisr=1
 	EXCEPTION_PROLOG_1
 	b	handle_page_fault_tramp_1
 #else	/* CONFIG_VMAP_STACK */
@@ -764,7 +756,7 @@ fast_hash_page_return:
 	/* DSI */
 	mtcr	r11
 	lwz	r11, THR11(r10)
-	mfspr	r10, SPRN_SPRG_SCRATCH0
+	mfspr	r10, SPRN_SPRG_SCRATCH2
 	rfi
 
 1:	/* ISI */