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

[Ard Biesheuvel]

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' ?