linux-kernel - Re: [PATCH 2/3] ARM: Move thread_info into task

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Date:   Tue, 07 Sep 2021 15:17:09 -0700
From:   Keith Packard <keithp@...thp.com>
To:     Ard Biesheuvel <ardb@...nel.org>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Abbott Liu <liuwenliang@...wei.com>,
        Alexander Sverdlin <alexander.sverdlin@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Anshuman Khandual <anshuman.khandual@....com>,
        Arnd Bergmann <arnd@...db.de>,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        Florian Fainelli <f.fainelli@...il.com>,
        Geert Uytterhoeven <geert+renesas@...der.be>,
        Hartley Sweeten <hsweeten@...ionengravers.com>,
        Jens Axboe <axboe@...nel.dk>, Jian Cai <jiancai@...gle.com>,
        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>,
        Manivannan Sadhasivam <mani@...nel.org>,
        Marc Zyngier <maz@...nel.org>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Miguel Ojeda <ojeda@...nel.org>,
        Mike Rapoport <rppt@...nel.org>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Nicolas Pitre <nico@...xnic.net>,
        Rob Herring <robh@...nel.org>,
        Russell King <linux@...linux.org.uk>,
        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 2/3] ARM: Move thread_info into task_struct (v7 only)

Ard Biesheuvel <ardb@...nel.org> writes:

> That is still a bit tricky, given that you now have to swap out the
> per-CPU offset when you migrate a task, and the generic code is not
> really set up for that.

Yeah, I decided to just leverage as much generic code as possible and
re-purposed the TPIDRPRW register for 'current', leaving the
per_cpu_offset data in memory. That makes the patch series pretty
compact, with a bunch of restructuring changes followed by fairly short
functional changes.

> I had a stab at this myself today (long boring day with no Internet connection).
>
> https://android-kvm.googlesource.com/linux/+log/refs/heads/ardb/arm32-ti-in-task
>
> It resembles your code in some places - I suppose we went on the same
> journey in a sense :-) We'll fix up the credits before this gets
> resubmitted.

This does look great, and preserves the optimization of keeping the
per_cpu_offset value in a register.

With the series I posted, getting the per_cpu_offset value requires
fetching the cpu value from task_struct and then using that to index the
per_cpu_offset arrray. I don't have a good feeling of how important this
optimization is? We could stick the per_cpu_offset value into
thread_info to avoid one fetch?

> Fixing the per-task stack protector plugin on top of these changes
> should be trivial but I need a coffee first.

I haven't even looked at that step yet :-)

-- 
-keith

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