Hi,

On 21.12.2021 14:34, Ard Biesheuvel wrote:

On Tue, 21 Dec 2021 at 12:15, Marek Szyprowski [off-list ref] wrote:

quoted

Hi Ard,

On 21.12.2021 11:44, Ard Biesheuvel wrote:

quoted

On Tue, 21 Dec 2021 at 11:39, Marek Szyprowski [off-list ref] wrote:

quoted

On 22.11.2021 10:28, Ard Biesheuvel wrote:

quoted

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@kernel.org>
Tested-by: Keith Packard <redacted>

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.

quoted

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


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