powerpc/32s: Reorganise DSI handler.

The part decidated to handling hash_page() is fully unneeded for
processors not having real hash pages like the 603.

Lets enlarge the content of the feature fixup, and provide
an alternative which jumps directly instead of getting NIPs.

Also, in preparation of VMAP stacks, the end of DSI handler has moved
to later in the code as it won't fit anymore once VMAP stacks
are there.

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/c31b22c91af8b011d0a4fd9e52ad6afb4b593f71.1576916812.git.christophe.leroy@c-s.fr

arch/powerpc/kernel/head_32.S

@@ -295,24 +295,20 @@ __secondary_hold_acknowledge:
 	DO_KVM  0x300
 DataAccess:
 	EXCEPTION_PROLOG
-	mfspr	r10,SPRN_DSISR
-	stw	r10,_DSISR(r11)
+	get_and_save_dar_dsisr_on_stack	r4, r5, r11
+BEGIN_MMU_FTR_SECTION
 #ifdef CONFIG_PPC_KUAP
-	andis.	r0,r10,(DSISR_BAD_FAULT_32S | DSISR_DABRMATCH | DSISR_PROTFAULT)@h
+	andis.	r0, r5, (DSISR_BAD_FAULT_32S | DSISR_DABRMATCH | DSISR_PROTFAULT)@h
 #else
-	andis.	r0,r10,(DSISR_BAD_FAULT_32S|DSISR_DABRMATCH)@h
+	andis.	r0, r5, (DSISR_BAD_FAULT_32S | DSISR_DABRMATCH)@h
 #endif
-	bne	1f			/* if not, try to put a PTE */
-	mfspr	r4,SPRN_DAR		/* into the hash table */
-	rlwinm	r3,r10,32-15,21,21	/* DSISR_STORE -> _PAGE_RW */
-BEGIN_MMU_FTR_SECTION
+	bne	handle_page_fault_tramp_2	/* if not, try to put a PTE */
+	rlwinm	r3, r5, 32 - 15, 21, 21		/* DSISR_STORE -> _PAGE_RW */
 	bl	hash_page
-END_MMU_FTR_SECTION_IFSET(MMU_FTR_HPTE_TABLE)
-1:	lwz	r5,_DSISR(r11)		/* get DSISR value */
-	mfspr	r4,SPRN_DAR
-	stw	r4, _DAR(r11)
-	EXC_XFER_LITE(0x300, handle_page_fault)
-
+	b	handle_page_fault_tramp_1
+FTR_SECTION_ELSE
+	b	handle_page_fault_tramp_2
+ALT_MMU_FTR_SECTION_END_IFSET(MMU_FTR_HPTE_TABLE)
 
 /* Instruction access exception. */
 	. = 0x400
@@ -642,6 +638,13 @@ END_MMU_FTR_SECTION_IFSET(MMU_FTR_NEED_DTLB_SW_LRU)
 
 	. = 0x3000
 
+handle_page_fault_tramp_1:
+	lwz	r4, _DAR(r11)
+	lwz	r5, _DSISR(r11)
+	/* fall through */
+handle_page_fault_tramp_2:
+	EXC_XFER_LITE(0x300, handle_page_fault)
+
 AltiVecUnavailable:
 	EXCEPTION_PROLOG
 #ifdef CONFIG_ALTIVEC