Re: [PATCH] fixup! signal: factor copy_siginfo_to_external32 from copy_siginfo_to_user32

From: Arnd Bergmann <arnd@arndb.de>
To: Christoph Hellwig <hch@lst.de>
Cc: Andrew Morton <akpm@linux-foundation.org>,
	Alexander Viro <viro@zeniv.linux.org.uk>,
	Jeremy Kerr <jk@ozlabs.org>,
	"Eric W . Biederman" <ebiederm@xmission.com>,
	linuxppc-dev <linuxppc-dev@lists.ozlabs.org>,
	Linux FS-devel Mailing List <linux-fsdevel@vger.kernel.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] fixup! signal: factor copy_siginfo_to_external32 from copy_siginfo_to_user32
Date: Wed, 29 Apr 2020 13:28:11 +0200	[thread overview]
Message-ID: <CAK8P3a1+DU+juB_SxAAK6WAMHwi7vGQS7T_Yw0Gvo4P4M8jggg@mail.gmail.com> (raw)
In-Reply-To: <20200429094201.GA2557@lst.de>

On Wed, Apr 29, 2020 at 11:42 AM Christoph Hellwig <hch@lst.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