[PATCH v4 7/7] ARM: implement support for vmap'ed stacks

[Marek Szyprowski]

Hi Krzysztof,

On 21.12.2021 11:42, Krzysztof Kozlowski wrote:
> On 21/12/2021 11:38, Marek Szyprowski wrote:
>> On 22.11.2021 10:28, Ard Biesheuvel wrote:
>>> Wire up the generic support for managing task stack allocations via vmalloc,
>>> and implement the entry code that detects whether we faulted because of a
>>> stack overrun (or future stack overrun caused by pushing the pt_regs array)
>>>
>>> While this adds a fair amount of tricky entry asm code, it should be
>>> noted that it only adds a TST + branch to the svc_entry path. The code
>>> implementing the non-trivial handling of the overflow stack is emitted
>>> out-of-line into the .text section.
>>>
>>> Since on ARM, we rely on do_translation_fault() to keep PMD level page
>>> table entries that cover the vmalloc region up to date, we need to
>>> ensure that we don't hit such a stale PMD entry when accessing the
>>> stack. So we do a dummy read from the new stack while still running from
>>> the old one on the context switch path, and bump the vmalloc_seq counter
>>> when PMD level entries in the vmalloc range are modified, so that the MM
>>> switch fetches the latest version of the entries.
>>>
>>> Note that we need to increase the per-mode stack by 1 word, to gain some
>>> space to stash a GPR until we know it is safe to touch the stack.
>>> However, due to the cacheline alignment of the struct, this does not
>>> actually increase the memory footprint of the struct stack array at all.
>>>
>>> Signed-off-by: Ard Biesheuvel <ardb at kernel.org>
>>> Tested-by: Keith Packard <keithpac at amazon.com>
>>
>> This patch landed recently in linux-next 20211220 as commit a1c510d0adc6
>> ("ARM: implement support for vmap'ed stacks"). Sadly it breaks
>> suspend/resume operation on all ARM 32bit Exynos SoCs. Probably the
>> suspend/resume related code must be updated somehow (it partially works
>> on physical addresses and disabled MMU), but I didn't analyze it yet. If
>> you have any hints, let me know.
> Maybe this one would help?
> https://lore.kernel.org/lkml/20211218085843.212497-2-cuigaosheng1@huawei.com/

I forgot to mention. I've already checked it and it doesn't change/fix 
anything. It also doesn't break the old (pre-a1c510d0adc) code though.

Best regards
-- 
Marek Szyprowski, PhD
Samsung R&D Institute Poland