[PATCH v3 25/60] arm64: head: Clear BSS and the kernel page tables in one go

Ard Biesheuvel
2023-03-07T14:10:37Z

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>
---
arch/arm64/kernel/head.S | 33 +++++++-------------
1 file changed, 11 insertions(+), 22 deletions(-)

diff --git a/arch/arm64/kernel/head.S b/arch/arm64/kernel/head.S
index 0fa44b3188c1e204..ade0cb99c8a83a3d 100644
--- a/arch/arm64/kernel/head.S
+++ b/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 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
@@ -780,6 +761,15 @@ 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
@@ -794,7 +784,6 @@ 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
--
2.39.2