linux-kernel - Re: [PATCH 4/7] ARM: Use hack from powerpc to get current cpu number

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Message-ID: <CAMj1kXE6woRq1Z1BDN3qKgXPyJkrePMgUmQHnFo1CMs-NyYt9A@mail.gmail.com>
Date:   Wed, 8 Sep 2021 09:45:23 +0200
From:   Ard Biesheuvel <ardb@...nel.org>
To:     Keith Packard <keithpac@...zon.com>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Abbott Liu <liuwenliang@...wei.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andrey Ryabinin <ryabinin.a.a@...il.com>,
        Anshuman Khandual <anshuman.khandual@....com>,
        Arnd Bergmann <arnd@...db.de>,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        Christoph Lameter <cl@...ux.com>,
        Dennis Zhou <dennis@...nel.org>,
        Geert Uytterhoeven <geert+renesas@...der.be>,
        Jens Axboe <axboe@...nel.dk>, Joe Perches <joe@...ches.com>,
        Kees Cook <keescook@...omium.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski@...onical.com>,
        Linus Walleij <linus.walleij@...aro.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Linux Memory Management List <linux-mm@...ck.org>,
        Manivannan Sadhasivam <mani@...nel.org>,
        Marc Zyngier <maz@...nel.org>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Mike Rapoport <rppt@...nel.org>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Nick Desaulniers <ndesaulniers@...ogle.com>,
        Nicolas Pitre <nico@...xnic.net>,
        Russell King <linux@...linux.org.uk>,
        Tejun Heo <tj@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Uwe Kleine-König 
        <u.kleine-koenig@...gutronix.de>,
        Valentin Schneider <valentin.schneider@....com>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        "Wolfram Sang (Renesas)" <wsa+renesas@...g-engineering.com>,
        YiFei Zhu <yifeifz2@...inois.edu>
Subject: Re: [PATCH 4/7] ARM: Use hack from powerpc to get current cpu number

On Wed, 8 Sept 2021 at 00:00, Keith Packard <keithpac@...zon.com> wrote:
>
> When we enable THREAD_INFO_IN_TASK, the cpu number will disappear from
> thread_info and reappear in task_struct. As we cannot include
> linux/sched.h in asm/smp.h, there's no way to use that struct type in
> the raw_smp_processor_id macro. Instead, a hack from the powerpc code
> is used. This pulls the TI_CPU offset out of asm-offsets.h and uses
> that to find the cpu value.
>
> Signed-off-by: Keith Packard <keithpac@...zon.com>
> ---
>  arch/arm/Makefile          |  8 ++++++++
>  arch/arm/include/asm/smp.h | 18 +++++++++++++++++-
>  2 files changed, 25 insertions(+), 1 deletion(-)
>
> diff --git a/arch/arm/Makefile b/arch/arm/Makefile
> index 415c3514573a..6752995d2914 100644
> --- a/arch/arm/Makefile
> +++ b/arch/arm/Makefile
> @@ -284,6 +284,14 @@ stack_protector_prepare: prepare0
>         $(eval GCC_PLUGINS_CFLAGS += $(SSP_PLUGIN_CFLAGS))
>  endif
>
> +ifdef CONFIG_SMP
> +prepare: task_cpu_prepare
> +
> +PHONY += task_cpu_prepare
> +task_cpu_prepare: prepare0
> +       $(eval KBUILD_CFLAGS += -D_TI_CPU=$(shell awk '{if ($$2 == "TI_CPU") print $$3;}' include/generated/asm-offsets.h))
> +endif
> +
>  all:   $(notdir $(KBUILD_IMAGE))
>
>
> diff --git a/arch/arm/include/asm/smp.h b/arch/arm/include/asm/smp.h
> index d43b64635d77..f77ba3753bc4 100644
> --- a/arch/arm/include/asm/smp.h
> +++ b/arch/arm/include/asm/smp.h
> @@ -15,7 +15,23 @@
>  # error "<asm/smp.h> included in non-SMP build"
>  #endif
>
> -#define raw_smp_processor_id() (current_thread_info()->cpu)
> +/*
> + * This is particularly ugly: it appears we can't actually get the
> + * definition of task_struct here, but we need access to the CPU this
> + * task is running on, which is stored in task_struct when
> + * THREAD_INFO_IN_TASK is set.  Instead of using task_struct we're
> + * using TI_CPU which is extracted from asm-offsets.h by kbuild to get
> + * the current processor ID.
> + *
> + * This also needs to be safeguarded when building asm-offsets.s
> + * because at that time TI_CPU is not defined yet.
> + */
> +#ifndef _TI_CPU
> +#define raw_smp_processor_id()         (0)
> +#else
> +#define raw_smp_processor_id() \
> +       (*(unsigned int *)((void *)current_thread_info() + _TI_CPU))
> +#endif
>
>  struct seq_file;
>

As I stated before, I would really like to avoid using this hack - I
don't think we need to.