Revert "powerpc/8xx: Always pin kernel text TLB"

This reverts commit bccc5898.

When STRICT_KERNEL_RWX is selected, EXEC memory must stop where
RW memory start. When pinning iTLBs it means an 8M alignment for
RW data start. That may be acceptable on boards with a lot of
memory but one of my supported boards only has 32 Mbytes and this
forced alignment leads to a waste of almost 4 Mbytes with is more
than 10% of the total memory.

So revert commit bccc5898 ("powerpc/8xx: Always pin kernel text
TLB") but don't restore previous behaviour in ITLB miss handler
as now kernel PGD entries are copied into each process PGDIR.
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/01b6780b860c8043b51a1ba9d83acfc6f2dde910.1724173828.git.christophe.leroy@csgroup.eu

arch/powerpc/kernel/head_8xx.S

@@ -587,6 +587,10 @@ start_here:
 	lis	r0, (MD_TWAM | MD_RSV4I)@h
 	mtspr	SPRN_MD_CTR, r0
 #endif
+#ifndef CONFIG_PIN_TLB_TEXT
+	li	r0, 0
+	mtspr	SPRN_MI_CTR, r0
+#endif
 #if !defined(CONFIG_PIN_TLB_DATA) && !defined(CONFIG_PIN_TLB_IMMR)
 	lis	r0, MD_TWAM@h
 	mtspr	SPRN_MD_CTR, r0
@@ -683,6 +687,7 @@ SYM_FUNC_START_LOCAL(initial_mmu)
 	blr
 SYM_FUNC_END(initial_mmu)
 
+#ifdef CONFIG_PIN_TLB
 _GLOBAL(mmu_pin_tlb)
 	lis	r9, (1f - PAGE_OFFSET)@h
 	ori	r9, r9, (1f - PAGE_OFFSET)@l
@@ -704,6 +709,7 @@ _GLOBAL(mmu_pin_tlb)
 	mtspr	SPRN_MD_CTR, r6
 	tlbia
 
+#ifdef CONFIG_PIN_TLB_TEXT
 	LOAD_REG_IMMEDIATE(r5, 28 << 8)
 	LOAD_REG_IMMEDIATE(r6, PAGE_OFFSET)
 	LOAD_REG_IMMEDIATE(r7, MI_SVALID | MI_PS8MEG | _PMD_ACCESSED)
@@ -724,6 +730,7 @@ _GLOBAL(mmu_pin_tlb)
 	bdnzt	lt, 2b
 	lis	r0, MI_RSV4I@h
 	mtspr	SPRN_MI_CTR, r0
+#endif
 
 	LOAD_REG_IMMEDIATE(r5, 28 << 8 | MD_TWAM)
 #ifdef CONFIG_PIN_TLB_DATA
@@ -783,3 +790,4 @@ _GLOBAL(mmu_pin_tlb)
 	mtspr	SPRN_SRR1, r10
 	mtspr	SPRN_SRR0, r11
 	rfi
+#endif

arch/powerpc/mm/nohash/8xx.c

@@ -177,7 +177,8 @@ int mmu_mark_initmem_nx(void)
 	if (!debug_pagealloc_enabled_or_kfence())
 		err = mmu_mapin_ram_chunk(boundary, einittext8, PAGE_KERNEL, false);
 
-	mmu_pin_tlb(block_mapped_ram, false);
+	if (IS_ENABLED(CONFIG_PIN_TLB_TEXT))
+		mmu_pin_tlb(block_mapped_ram, false);
 
 	return err;
 }

arch/powerpc/platforms/8xx/Kconfig

@@ -195,6 +195,13 @@ config PIN_TLB_IMMR
 	  CONFIG_PIN_TLB_DATA is also selected, it will reduce
 	  CONFIG_PIN_TLB_DATA to 24 Mbytes.
 
+config PIN_TLB_TEXT
+	bool "Pinned TLB for TEXT"
+	depends on PIN_TLB
+	default y
+	help
+	  This pins kernel text with 8M pages.
+
 endmenu
 
 endmenu