Date: Wed, 5 Oct 2016 22:47:40 +0100
From: Mark Rutland <mark.rutland@arm.com>
To: Russell King - ARM Linux <linux@armlinux.org.uk>
Cc: Fredrik =?utf-8?Q?Markstr=C3=B6m?= <fredrik.markstrom@gmail.com>,
        Robin Murphy <robin.murphy@arm.com>, Kees Cook <keescook@chromium.org>,
        Arnd Bergmann <arnd@arndb.de>,
        Ard Biesheuvel <ard.biesheuvel@linaro.org>,
        Linus Walleij <linus.walleij@linaro.org>,
        Nicolas Pitre <nico@linaro.org>, Will Deacon <will.deacon@arm.com>,
        kristina.martsenko@arm.com, linux-kernel@vger.kernel.org,
        Masahiro Yamada <yamada.masahiro@socionext.com>,
        Chris Brandt <chris.brandt@renesas.com>,
        Michal Marek <mmarek@suse.com>, Zhaoxiu Zeng <zhaoxiu.zeng@gmail.com>,
        linux-arm-kernel@lists.infradead.org,
        Jonathan Austin <jonathan.austin@arm.com>
Subject: Re: [PATCH v2] arm: Added support for getcpu() vDSO using TPIDRURW
Message-ID: <20161005214729.GA27121@remoulade>
References: <1475589000-29315-1-git-send-email-fredrik.markstrom@gmail.com>
 <1475595363-4272-1-git-send-email-fredrik.markstrom@gmail.com>
 <20161004170741.GC29008@leverpostej>
 <CAKdL+dS4_My6hyMEGNc65mzDapia_tMiVzZ9DMw=ddZM+XiwAw@mail.gmail.com>
 <CAKdL+dTGDqgpnMTkAj=N4cY-cZF_U+bkH1v1vUA4umZoSbWHKQ@mail.gmail.com>
 <50e025e0-7052-9b15-3b3e-36d1d9dfd695@arm.com>
 <CAKdL+dQH=9C2aGf7ys5-vXM7pkdPYUQ8xYWLipwVbABOz09f1g@mail.gmail.com>
 <20161005204452.GB26721@remoulade>
 <20161005210137.GS1041@n2100.armlinux.org.uk>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20161005210137.GS1041@n2100.armlinux.org.uk>
User-Agent: Mutt/1.5.24 (2015-08-30)
Sender: linux-kernel-owner@vger.kernel.org
Content-Length: 1559
Lines: 36

On Wed, Oct 05, 2016 at 10:01:38PM +0100, Russell King - ARM Linux wrote:
> On Wed, Oct 05, 2016 at 09:44:53PM +0100, Mark Rutland wrote:
> > The zeroing case is similar to the restartable sequences design. So that's
> > probably worth looking into.
> 
> You're sending mixed messages: in your previous message, you said:
> 
>   Arguably, someone could have (ab)used TPIDRURW between commits 6a1c531
>   and a4780ad to detect context switches, but in practice they don't
>   appear to have, and we know of an established user relying on the
>   current behaviour.
> 
>   For better or worse, the current behaviour is ABI.
> 
> Now you're suggesting that we could go back to the case where the
> register is zeroed.

Sorry; clumsy wording on my behalf.

I meant that functionality-wise, restartable sequences had similar behaviour to
the zeroing case (without touching TPIDRURW at all) and were probably worth
looking at. I did not intend to suggest that we should go pack to case where
TPIDRURW was zeroed.

> Well, the fact is that we _can_ change the TPIDRURW behaviour - we just
> need to be careful about how we change it.  Eg, we _could_ introduce a
> per-process flag which indicates that we want some other behaviour from
> TPIDRURW such as zeroing it on context switches.  The default would be
> to preserve the existing behaviour as doing anything else breaks
> existing programs.  The problem there is finding an acceptable way to
> control such a flag from userspace (eg, prctl, syscall, etc).

Sure. Something like that could work.

Thanks,
Mark.