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

From: Marek Szyprowski <m.szyprowski@samsung.com>
To: Krzysztof Kozlowski <krzk@kernel.org>,
	Ard Biesheuvel <ardb@kernel.org>,
	linux-arm-kernel@lists.infradead.org
Cc: Russell King <linux@armlinux.org.uk>,
	Nicolas Pitre <nico@fluxnic.net>, Arnd Bergmann <arnd@arndb.de>,
	Kees Cook <keescook@chromium.org>,
	Keith Packard <keithpac@amazon.com>,
	Linus Walleij <linus.walleij@linaro.org>,
	Nick Desaulniers <ndesaulniers@google.com>,
	Tony Lindgren <tony@atomide.com>,
	'Linux Samsung SOC' <linux-samsung-soc@vger.kernel.org>
Subject: Re: [PATCH v4 7/7] ARM: implement support for vmap'ed stacks
Date: Tue, 21 Dec 2021 11:46:43 +0100	[thread overview]
Message-ID: <fbd24aa1-4e04-feff-9125-482db22a05ef@samsung.com> (raw)
In-Reply-To: <b484dea4-da88-3e09-64c5-e1b5bb6cd639@kernel.org>

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