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

Ryan Roberts
2023-04-17T14:10:08Z

On 17/04/2023 15:02, Ard Biesheuvel wrote:
> On Mon, 17 Apr 2023 at 16:00, Ryan Roberts <ryan.roberts@arm.com> wrote:
>>
>> On 07/03/2023 14:04, Ard Biesheuvel wrote:
>>> 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
>>
>> Is it possible to add an assert somewhere (or at the very least a comment in
>> vmlinux.lds.S) to ensure that nothing gets inserted between the BSS and the page
>> tables? It feels a bit fragile otherwise.
>>
>
> I'm not sure that matters. The contents are not covered by the loaded
> image so they are undefined otherwise in any case.

OK, so you couldn't accidentally zero anything in the image. But it could
represent a performance regression if something big was added between them that
doesn't need to be zeroed. All hypothetical, but this is currently an unstated
assumption that I think is worth stating at least as a comment in the linker script.

>
>> I also wonder what's the point in calling __pi_memset() from here? Why not just
>> do it all in C?
>>
>
> That happens in one of the subsequent patches.

Ahh, cheers... Haven't got that far yet. (very impressive that you immediately
knew that given you posted the series 6 weeks ago! I usually can't remember what
I did yesterday ;-)