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

[Ryan Roberts]

On 17/04/2023 15:02, Ard Biesheuvel wrote:
> On Mon, 17 Apr 2023 at 16:00, Ryan Roberts <ryan.roberts@xxxxxxx> 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@xxxxxxxxxx>
>>> ---
>>>  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 ;-)