linux-kernel - Re: linux-next: Tree for Jul 31

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Date:   Fri, 2 Aug 2019 09:44:43 +0300
From:   Ard Biesheuvel <ard.biesheuvel@...aro.org>
To:     Herbert Xu <herbert@...dor.apana.org.au>
Cc:     Stephen Rothwell <sfr@...b.auug.org.au>,
        Heiko Carstens <heiko.carstens@...ibm.com>,
        Linux Next Mailing List <linux-next@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-s390 <linux-s390@...r.kernel.org>,
        Harald Freudenberger <freude@...ux.ibm.com>,
        Patrick Steuer <steuer@...ux.ibm.com>,
        Ondrej Mosnacek <omosnace@...hat.com>
Subject: Re: linux-next: Tree for Jul 31 - s390 crypto build breakage

On Fri, 2 Aug 2019 at 06:14, Herbert Xu <herbert@...dor.apana.org.au> wrote:
>
> Hi Stephen:
>
> On Fri, Aug 02, 2019 at 10:20:19AM +1000, Stephen Rothwell wrote:
> >
> > It might be time to revert all this series and try again.  The
> > implementation seems to have not been well thought through from a kernel
> > building point of view.  For a start the two commits
> >
> >   7cdc0ddbf74a ("crypto: aegis128 - add support for SIMD acceleration")
> >   ecc8bc81f2fb ("crypto: aegis128 - provide a SIMD implementation based on NEON intrinsics")
>
> I think the idea was that it would get optimised out if the
> implementation is absent which is why it was meant to work in
> this order.  But oviously as we have found out this didn't work.
>
> Ard, I think relying on the compiler to optimise something out based
> on an assignment within an if statement is just too error-prone.
> We'll need a different mechanism for this.
>

Indeed. This is definitely something I tested, and it appears to be
dependent on the GCC version.

> For now I'm going to back out those two specific patches as the
> rest seem to be valid by themselves.
>

OK. I will adopt this mechanism [0] after all and resubmit, once I get
confirmation from either Voldis or Heiko that this makes the issue go
away (given that my local GCC does not reproduce the issue)

[0] https://lore.kernel.org/linux-crypto/20190729074434.21064-1-ard.biesheuvel@linaro.org/