linux-kernel - Re: [PATCH v2] arm: Added support for getcpu() vDSO using TPIDRURW

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Message-ID: <CAKdL+dTYR7Ch9uwyzZGG_wz533pth_+4CkAzYtzaCVn5bF20ig@mail.gmail.com>
Date:   Wed, 5 Oct 2016 23:37:06 +0200
From:   Fredrik Markström <fredrik.markstrom@...il.com>
To:     Mark Rutland <mark.rutland@....com>
Cc:     Robin Murphy <robin.murphy@....com>,
        Kees Cook <keescook@...omium.org>,
        Arnd Bergmann <arnd@...db.de>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Linus Walleij <linus.walleij@...aro.org>,
        Nicolas Pitre <nico@...aro.org>,
        Will Deacon <will.deacon@....com>,
        Russell King <linux@...linux.org.uk>,
        kristina.martsenko@....com, linux-kernel@...r.kernel.org,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Chris Brandt <chris.brandt@...esas.com>,
        Michal Marek <mmarek@...e.com>,
        Zhaoxiu Zeng <zhaoxiu.zeng@...il.com>,
        linux-arm-kernel@...ts.infradead.org,
        Jonathan Austin <jonathan.austin@....com>
Subject: Re: [PATCH v2] arm: Added support for getcpu() vDSO using TPIDRURW

On Wed, Oct 5, 2016 at 10:44 PM, Mark Rutland <mark.rutland@....com> wrote:
> On Wed, Oct 05, 2016 at 08:00:38PM +0000, Fredrik Markström wrote:
>> On Wed, Oct 5, 2016 at 7:48 PM Robin Murphy <robin.murphy@....com> wrote:
>> As far as I understand TPIDRURW isn't anything else then an architecture
>> specific piece of tls since the last patch, possibly slightly faster then a
>> "__thread u32 x;"
>>
>> The irony is that the two different ways it was handled earlier (not context
>> switched or always set to zero on swap in) would have made it useful for this
>> purpose.
>
> The "not context switched" case was also arbitrarily corrupted, and could not
> have been relied upon.

Ok, I missed that, sorry !

>
> The zeroing case is similar to the restartable sequences design. So that's
> probably worth looking into.

Ok, I'm starting to believe my best bet is to hope that those make it
into the kernel
eventually, until then I'll probably just go with a local solution.

/Fredrik

>
> Thanks,
> Mark.