linux-kernel - Re: [PATCH v2 18/18] arm64: select ARCH_SUPPORTS_LTO

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Date:   Thu, 16 Nov 2017 18:34:17 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     Sami Tolvanen <samitolvanen@...gle.com>,
        Will Deacon <will.deacon@....com>,
        Alex Matveev <alxmtvv@...il.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Greg Hackmann <ghackmann@...gle.com>,
        Kees Cook <keescook@...omium.org>,
        linux-arm-kernel@...ts.infradead.org,
        Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Mark Rutland <mark.rutland@....com>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Maxim Kuvyrkov <maxim.kuvyrkov@...aro.org>,
        Michal Marek <michal.lkml@...kovi.net>,
        Yury Norov <ynorov@...iumnetworks.com>,
        Matthias Kaehlcke <mka@...omium.org>,
        paulmck@...ux.vnet.ibm.com
Subject: Re: [PATCH v2 18/18] arm64: select ARCH_SUPPORTS_LTO_CLANG

On Thu, Nov 16, 2017 at 09:16:49AM -0800, Nick Desaulniers wrote:
> On Thu, Nov 16, 2017 at 8:59 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> > On Thu, Nov 16, 2017 at 08:50:41AM -0800, Nick Desaulniers wrote:
> >> On Thu, Nov 16, 2017 at 8:30 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> >>
> >> > Ideally we'd get the toolchain people to commit to supporting the kernel
> >> > memory model along side the C11 one. That would help a ton.
> >>
> >> Does anyone from the kernel side participate in the C standardization process?
> >
> > Yes, Paul McKenney and Will Deacon. Doesn't mean these two can still be
> > reconciled though. From what I understand C11 (and onwards) are
> > incompatible with the kernel model on a number of subtle points.
> 
> It would be good to have these incompatibilities written down, then
> for the sake of argument, they can be cited both for discussions on
> LKML and in the C standardization process.  For example, a running
> list in Documentation/ or something would make it so that anyone could
> understand and cite current issues with the latest C standard.

Will should be able to produce this list; I know he's done before, I
just can't find it -- my Google-foo isn't strong today.

> I don't understand why we'd block patches for enabling experimental
> features.  We've been running this patch-set on actual devices for
> months and would love to provide them to the community for further
> testing.  If bugs are found, then there's more evidence to bring to
> the C standards committee.  Otherwise we're shutting down feature
> development for the sake of potential bugs in a C standard we're not
> even using.

So the problem is that its very very hard (and painful) to find these
bugs. Getting the tools people to comment on these specific
optimizations would really help lots.