powerpc/32: Set an IBAT covering up to _einittext during init

Always set an IBAT covering up to _einittext during init because when
CONFIG_MODULES is not selected there is no reason to have an exception
handler for kernel instruction TLB misses.

It implies DBAT and IBAT are now totaly independent, IBATs are set
by setibat() and DBAT by setbat().

This allows to revert commit 9bb162fa ("powerpc/603: Fix
boot failure with DEBUG_PAGEALLOC and KFENCE")
Reported-by: Maxime Bizon <mbizon@freebox.fr>
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/ce7f04a39593934d9b1ee68c69144ccd3d4da4a1.1655202804.git.christophe.leroy@csgroup.eu

arch/powerpc/kernel/head_book3s_32.S

@@ -418,14 +418,14 @@ InstructionTLBMiss:
  */
 	/* Get PTE (linux-style) and check access */
 	mfspr	r3,SPRN_IMISS
-#if defined(CONFIG_MODULES) || defined(CONFIG_DEBUG_PAGEALLOC) || defined(CONFIG_KFENCE)
+#ifdef CONFIG_MODULES
 	lis	r1, TASK_SIZE@h		/* check if kernel address */
 	cmplw	0,r1,r3
 #endif
 	mfspr	r2, SPRN_SDR1
 	li	r1,_PAGE_PRESENT | _PAGE_ACCESSED | _PAGE_EXEC | _PAGE_USER
 	rlwinm	r2, r2, 28, 0xfffff000
-#if defined(CONFIG_MODULES) || defined(CONFIG_DEBUG_PAGEALLOC) || defined(CONFIG_KFENCE)
+#ifdef CONFIG_MODULES
 	bgt-	112f
 	lis	r2, (swapper_pg_dir - PAGE_OFFSET)@ha	/* if kernel address, use */
 	li	r1,_PAGE_PRESENT | _PAGE_ACCESSED | _PAGE_EXEC

arch/powerpc/mm/book3s32/mmu.c

@@ -159,7 +159,10 @@ unsigned long __init mmu_mapin_ram(unsigned long base, unsigned long top)
 {
 	unsigned long done;
 	unsigned long border = (unsigned long)__init_begin - PAGE_OFFSET;
+	unsigned long size;
 
+	size = roundup_pow_of_two((unsigned long)_einittext - PAGE_OFFSET);
+	setibat(0, PAGE_OFFSET, 0, size, PAGE_KERNEL_X);
 
 	if (debug_pagealloc_enabled_or_kfence()) {
 		pr_debug_once("Read-Write memory mapped without BATs\n");
@@ -245,10 +248,9 @@ void mmu_mark_rodata_ro(void)
 }
 
 /*
- * Set up one of the I/D BAT (block address translation) register pairs.
+ * Set up one of the D BAT (block address translation) register pairs.
  * The parameters are not checked; in particular size must be a power
  * of 2 between 128k and 256M.
- * On 603+, only set IBAT when _PAGE_EXEC is set
  */
 void __init setbat(int index, unsigned long virt, phys_addr_t phys,
 		   unsigned int size, pgprot_t prot)
@@ -284,10 +286,6 @@ void __init setbat(int index, unsigned long virt, phys_addr_t phys,
 		/* G bit must be zero in IBATs */
 		flags &= ~_PAGE_EXEC;
 	}
-	if (flags & _PAGE_EXEC)
-		bat[0] = bat[1];
-	else
-		bat[0].batu = bat[0].batl = 0;
 
 	bat_addrs[index].start = virt;
 	bat_addrs[index].limit = virt + ((bl + 1) << 17) - 1;