linux-kernel - Re: [PATCH] fixup! signal: factor copy_siginfo_to_external32 from copy_siginfo_to

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Message-ID: <CAK8P3a1+DU+juB_SxAAK6WAMHwi7vGQS7T_Yw0Gvo4P4M8jggg@mail.gmail.com>
Date:   Wed, 29 Apr 2020 13:28:11 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Christoph Hellwig <hch@....de>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Jeremy Kerr <jk@...abs.org>,
        "Eric W . Biederman" <ebiederm@...ssion.com>,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        Linux FS-devel Mailing List <linux-fsdevel@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] fixup! signal: factor copy_siginfo_to_external32 from copy_siginfo_to_user32

On Wed, Apr 29, 2020 at 11:42 AM Christoph Hellwig <hch@....de> wrote:
>
> On Wed, Apr 29, 2020 at 10:07:11AM +0200, Arnd Bergmann wrote:
> > > What do you think of this version?  This one always overrides
> > > copy_siginfo_to_user32 for the x86 compat case to keep the churn down,
> > > and improves the copy_siginfo_to_external32 documentation a bit.
> >
> > Looks good to me. I preferred checking for X32 explicitly (so we can
> > find and kill off the #ifdef if we ever remove X32 for good), but there is
> > little difference in the end.
>
> Is there any realistic chance we'll get rid of x32?

When we discussed it last year, there were a couple of users that replied
saying they actively use it for a full system, and some others said they run
specific programs built as x32 as it results in much faster (10% to 20%)
execution of the same binaries compared to either i686 or x86_64.

I expect both of these to get less common over time as stuff bitrots
and more of the workloads that benefit most from the higher
performance (cross-compilers, hpc) run out of virtual address space.
Debian popcon numbers are too small to be reliable but they do show
a trend at https://popcon.debian.org/stat/sub-x32.png

I would just ask again every few years, and eventually we'll decide
it's not worth keeping any more. I do expect most 32-bit machines
to stop getting kernel updates before 2030 and we can probably
remove a bunch of architectures including x32 before then, though
at least armv7 users will have to get kernel updates for substantially
longer.

      Arnd