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

[Marek Szyprowski]

Hi,

On 21.12.2021 14:34, Ard Biesheuvel wrote:
> On Tue, 21 Dec 2021 at 12:15, Marek Szyprowski <m.szyprowski at samsung.com> wrote:
>> Hi Ard,
>>
>> On 21.12.2021 11:44, Ard Biesheuvel wrote:
>>> On Tue, 21 Dec 2021 at 11:39, Marek Szyprowski <m.szyprowski at samsung.com> 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.
>>>>
>>> Are there any such systems in KernelCI? We caught a suspend/resume
>>> related issue in development, which is why the hunk below was added.
>>
>> I think that some Exynos-based Odroids (U3 and XU3) were some time ago
>> available in KernelCI, but I don't know if they are still there.
>>
>>
>>> In general, any virt-to-phys translation involving and address on the
>>> stack will become problematic.
>>>
>>> Could you please confirm whether the issue persists with the patch
>>> applied but with CONFIG_VMAP_STACK turned off? Just so we know we are
>>> looking in the right place?
>>
>> I've just checked. After disabling CONFIG_VMAP_STACK suspend/resume
>> works fine both on commit a1c510d0adc6 and linux-next 20211220.
>>
> Thanks. Any other context you can provide beyond 'does not work' ?

Well, the board properly suspends, but it doesn't wake then (tested 
remotely with rtcwake command). So far I cannot provide anything more.

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