linux-kernel - Re: Can we drop upstream Linux x32 support?

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Date:   Thu, 13 Dec 2018 22:38:52 +0800
From:   Olof Johansson <olof@...om.net>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     luto@...nel.org, x86@...nel.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-api@...r.kernel.org, "H. Peter Anvin" <hpa@...or.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Borislav Petkov <bp@...en8.de>, fweimer@...hat.com,
        Mike Frysinger <vapier@...too.org>, hjl.tools@...il.com,
        dalias@...c.org, x32@...ldd.debian.org,
        Arnd Bergmann <arnd@...db.de>,
        Will Deacon <will.deacon@....com>,
        Catalin Marinas <catalin.marinas@....com>
Subject: Re: Can we drop upstream Linux x32 support?

On Tue, Dec 11, 2018 at 9:40 AM Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
> On Mon, Dec 10, 2018 at 5:23 PM Andy Lutomirski <luto@...nel.org> wrote:
> >
> > I'm seriously considering sending a patch to remove x32 support from
> > upstream Linux.  Here are some problems with it:
>
> I talked to Arnd (I think - we were talking about all the crazy ABI's,
> but maybe it was with somebody else) about exactly this in Edinburgh.
>
> Apparently the main real use case is for extreme benchmarking. It's
> the only use-case where the complexity of maintaining a whole
> development environment and distro is worth it, it seems. Apparently a
> number of Spec submissions have been done with the x32 model.
>
> I'm not opposed to trying to sunset the support, but let's see who complains..

I'm just a single user. I do rely on it though, FWIW.

I hadn't finished my benchmarking in Edinburgh, but for my new machine
that does kernel builds 24/7, I ended up going with x32 userspace (in
a container).

Main reason is that it's a free ~10% improvement in runtime over
64-bit. I.e. GCC-as-a-workload is quite a bit faster as x32,
supposedly mostly due to smaller D cache footprints (I ran out of
cycles to tinker with back and forth perf data collection and settled
down on just running it).

Running classic 32-bit (i386? i686? whatever it's called) is about
half as good. I.e. even then I get a ~5% performance win. Less than
x32, but still better than 64-bit userspace.

-Olof