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From: Arnd Bergmann <arnd@arndb.de>
To: Ard Biesheuvel <ardb@kernel.org>
Cc: Linux ARM <linux-arm-kernel@lists.infradead.org>,
	 Russell King - ARM Linux <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>
Subject: Re: [PATCH v2 00/12] ARM: enable IRQ stacks and vmap'ed stacks for UP
Date: Tue, 30 Nov 2021 15:08:09 +0100	[thread overview]
Message-ID: <CAK8P3a2_+JuR85Pcp4U3ypk8FLj5KpnFt1u-k_qZae3iD2-KtQ@mail.gmail.com> (raw)
In-Reply-To: <20211130125901.3054-1-ardb@kernel.org>

On Tue, Nov 30, 2021 at 1:58 PM Ard Biesheuvel <ardb@kernel.org> wrote:
>
> Enable the use of the TLS register to hold the 'current' pointer for all
> configurations that can support it, including non-SMP ones that target
> v6k or later CPUs, and multi-platform SMP ones that also support v6
> based UP systems.
>
> The remaining configurations are all strictly UP, which means we can
> switch to a global variable to hold the current pointer. By doing this,
> we can enable THREAD_INFO_IN_TASK, which moves thread info off the
> stack, protecting it from overflows. It also permits us to enable IRQ
> stacks and vmap'ed stacks for UP configurations as well.
>
> Supporting v6 cores without SMP extensions in SMP configurations (e.g.,
> omap2plus_defconfig or imx_v6_v7_defconfig) makes this a bit tricky, and
> this is a feature we may consider dropping entirely in the future. But
> for the time being, we can support this mode as well.
>
> The accesses to the global variable holding 'current' are constructed in
> a way that ensures that no literal pool accesses (and associated D-cache
> misses) are needed unless the access is from a module and module PLTs
> are enabled. This means that accessing 'current' is just as costly as
> before, as it used to require some arithmetic involving the stack
> pointer and a load from the thread_info::task field.
>
> However, accessing thread_info itself now also involves a load, although
> it should be noted that all thread_info and current accesses now go via
> the same variable, which is therefore expected to be hot in the caches
> at all times.

Most of your assembler changes are beyond my understanding of that
code, but everything that I do understand moves the code in the right
direction, this looks really good overall.

I'll add this to my randconfig compile test setup to see if that finds any
corner cases you may have missed.

        Arnd

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      parent reply	other threads:[~2021-11-30 14:16 UTC|newest]

Thread overview: 23+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-11-30 12:58 [PATCH v2 00/12] ARM: enable IRQ stacks and vmap'ed stacks for UP Ard Biesheuvel
2021-11-30 12:58 ` [PATCH v2 01/12] ARM: riscpc: use GENERIC_IRQ_MULTI_HANDLER Ard Biesheuvel
2021-11-30 12:58 ` [PATCH v2 02/12] ARM: footbridge: " Ard Biesheuvel
2021-12-04 23:42   ` Linus Walleij
2021-11-30 12:58 ` [PATCH v2 03/12] ARM: iop32x: offset IRQ numbers by 1 Ard Biesheuvel
2021-12-04 23:43   ` Linus Walleij
2021-11-30 12:58 ` [PATCH v2 04/12] ARM: iop32x: use GENERIC_IRQ_MULTI_HANDLER Ard Biesheuvel
2021-11-30 13:29   ` Arnd Bergmann
2021-11-30 12:58 ` [PATCH v2 05/12] ARM: remove old-style irq entry Ard Biesheuvel
2021-12-04 23:45   ` Linus Walleij
2021-11-30 12:58 ` [PATCH v2 06/12] ARM: entry: preserve thread_info pointer in switch_to Ard Biesheuvel
2021-11-30 12:58 ` [PATCH v2 07/12] ARM: module: implement support for PC-relative group relocations Ard Biesheuvel
2021-11-30 12:58 ` [PATCH v2 08/12] ARM: assembler: add optimized ldr/str macros to load variables from memory Ard Biesheuvel
2021-11-30 12:58 ` [PATCH v2 09/12] ARM: percpu: add SMP_ON_UP support Ard Biesheuvel
2021-11-30 15:12   ` Russell King (Oracle)
2021-11-30 15:45     ` Ard Biesheuvel
2021-11-30 12:58 ` [PATCH v2 10/12] ARM: use TLS register for 'current' on !SMP as well Ard Biesheuvel
2021-11-30 14:05   ` Arnd Bergmann
2021-11-30 12:59 ` [PATCH v2 11/12] ARM: smp: defer TPIDRURO update for SMP v6 configurations too Ard Biesheuvel
2021-11-30 12:59 ` [PATCH v2 12/12] ARM: implement THREAD_INFO_IN_TASK for uniprocessor systems Ard Biesheuvel
2021-11-30 13:43   ` Arnd Bergmann
2021-11-30 13:54     ` Ard Biesheuvel
2021-11-30 14:08 ` Arnd Bergmann [this message]

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