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

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Date:   Wed, 12 Dec 2018 10:04:53 +0100
From:   Arnd Bergmann <arnd@...db.de>
To:     tg@...bsd.de
Cc:     Andy Lutomirski <luto@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux API <linux-api@...r.kernel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Borislav Petkov <bp@...en8.de>,
        Florian Weimer <fweimer@...hat.com>, vapier@...too.org,
        "H.J. Lu" <hjl.tools@...il.com>, Rich Felker <dalias@...c.org>,
        x32@...ldd.debian.org, Will Deacon <will.deacon@....com>,
        Catalin Marinas <catalin.marinas@....com>
Subject: Re: Can we drop upstream Linux x32 support?

On Wed, Dec 12, 2018 at 3:38 AM Thorsten Glaser <tg@...bsd.de> wrote:
>
> Andy Lutomirski dixit:
>
> >That’s the thing, though: the whole generic kernel compat
> >infrastructure assumes there are at most two ABIs: native and, if
> >enabled and relevant, compat. x32 breaks this entirely.
>
> MIPS had o32, n32, n64 since like forever.

o32 and n32 are practically the same, the only difference on the
syscall ABI that I can see are the actual syscall numbers, and
the 'struct sigcontext' definition.

> ARM has old ABI, EABI and now 64-bit.

arm64 intentionally did not attempt to support OABI user space
because of this, and as I said the ilp32 ABI follows what MIPS
n32 does using the same data structures as aarch32 (corresponding
to mips o32).

> >How hard would it be to have __attribute__((ilp64)), with an optional
> >warning if any embedded structs are not ilp64?  This plus a wrapper to
>
> You mean LP64. Impossible, because LP64 vs. ILP32 is not the only
> difference between amd64 and x32.

I think the above is what Intel's compiler does, and similar to what they
do for mixing big-endian and little-endian code (!). Generally possible yes,
but a lot of work, as well as error-prone and not particular appealing for
the GNU toolchain IMHO.

        Arnd