arm64: head: Clear BSS and the kernel page tables in one go

We will move the CPU feature overrides into BSS in a subsequent patch,
and this requires that BSS is zeroed before the feature override
detection code runs. So let's map BSS read-write in the ID map, and zero
it via this mapping.

Since the kernel page tables are right next to it, and also zeroed via
the ID map, let's drop the separate clear_page_tables() function, and
just zero everything in one go.
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Link: https://lore.kernel.org/r/20240214122845.2033971-51-ardb+git@google.comSigned-off-by: Catalin Marinas <catalin.marinas@arm.com>

arch/arm64/kernel/head.S

@@ -177,17 +177,6 @@ SYM_CODE_START_LOCAL(preserve_boot_args)
 	ret
 SYM_CODE_END(preserve_boot_args)
 
-SYM_FUNC_START_LOCAL(clear_page_tables)
-	/*
-	 * Clear the init page tables.
-	 */
-	adrp	x0, init_pg_dir
-	adrp	x1, init_pg_end
-	sub	x2, x1, x0
-	mov	x1, xzr
-	b	__pi_memset			// tail call
-SYM_FUNC_END(clear_page_tables)
-
 /*
  * Macro to populate page table entries, these entries can be pointers to the next level
  * or last level entries pointing to physical memory.
@@ -386,9 +375,9 @@ SYM_FUNC_START_LOCAL(create_idmap)
 
 	map_memory x0, x1, x3, x6, x7, x3, IDMAP_PGD_ORDER, x10, x11, x12, x13, x14, EXTRA_SHIFT
 
-	/* Remap the kernel page tables r/w in the ID map */
+	/* Remap BSS and the kernel page tables r/w in the ID map */
 	adrp	x1, _text
-	adrp	x2, init_pg_dir
+	adrp	x2, __bss_start
 	adrp	x3, _end
 	bic	x4, x2, #SWAPPER_BLOCK_SIZE - 1
 	mov_q	x5, SWAPPER_RW_MMUFLAGS
@@ -489,14 +478,6 @@ SYM_FUNC_START_LOCAL(__primary_switched)
 	mov	x0, x20
 	bl	set_cpu_boot_mode_flag
 
-	// Clear BSS
-	adr_l	x0, __bss_start
-	mov	x1, xzr
-	adr_l	x2, __bss_stop
-	sub	x2, x2, x0
-	bl	__pi_memset
-	dsb	ishst				// Make zero page visible to PTW
-
 #if VA_BITS > 48
 	adr_l	x8, vabits_actual		// Set this early so KASAN early init
 	str	x25, [x8]			// ... observes the correct value
@@ -782,6 +763,15 @@ SYM_FUNC_START_LOCAL(__primary_switch)
 	adrp	x1, reserved_pg_dir
 	adrp	x2, init_idmap_pg_dir
 	bl	__enable_mmu
+
+	// Clear BSS
+	adrp	x0, __bss_start
+	mov	x1, xzr
+	adrp	x2, init_pg_end
+	sub	x2, x2, x0
+	bl	__pi_memset
+	dsb	ishst				// Make zero page visible to PTW
+
 #ifdef CONFIG_RELOCATABLE
 	adrp	x23, KERNEL_START
 	and	x23, x23, MIN_KIMG_ALIGN - 1
@@ -796,7 +786,6 @@ SYM_FUNC_START_LOCAL(__primary_switch)
 	orr	x23, x23, x0			// record kernel offset
 #endif
 #endif
-	bl	clear_page_tables
 	bl	create_kernel_mapping
 
 	adrp	x1, init_pg_dir

arch/arm64/kernel/vmlinux.lds.S

@@ -311,12 +311,15 @@ SECTIONS
 	__pecoff_data_rawsize = ABSOLUTE(. - __initdata_begin);
 	_edata = .;
 
+	/* start of zero-init region */
 	BSS_SECTION(SBSS_ALIGN, 0, 0)
 
 	. = ALIGN(PAGE_SIZE);
 	init_pg_dir = .;
 	. += INIT_DIR_SIZE;
 	init_pg_end = .;
+	/* end of zero-init region */
+
 #ifdef CONFIG_RELOCATABLE
 	. += SZ_4K;		/* stack for the early relocation code */
 	early_init_stack = .;