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Date: Sat, 14 Sep 2019 12:22:02 +0200
From: Michal =?UTF-8?B?U3VjaMOhbmVr?= <msuchanek@suse.de>
To: Michael Ellerman <mpe@ellerman.id.au>
Subject: Re: [PATCH v7 5/6] powerpc/64: Make COMPAT user-selectable disabled
 on littleendian by default.
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Cc: Michael Neuling <mikey@neuling.org>,
 Madhavan Srinivasan <maddy@linux.vnet.ibm.com>, Arnd Bergmann <arnd@arndb.de>,
 David Hildenbrand <david@redhat.com>,
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 Diana Craciun <diana.craciun@nxp.com>, Hari Bathini <hbathini@linux.ibm.com>,
 Paul Mackerras <paulus@samba.org>, "Eric W.
 Biederman" <ebiederm@xmission.com>,
 Andrew Donnellan <andrew.donnellan@au1.ibm.com>,
 Breno Leitao <leitao@debian.org>, Firoz Khan <firoz.khan@linaro.org>,
 Thomas Gleixner <tglx@linutronix.de>, linuxppc-dev@lists.ozlabs.org,
 Andrew Morton <akpm@linux-foundation.org>,
 Allison Randal <allison@lohutok.net>, Joel Stanley <joel@jms.id.au>
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 <linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org>

On Tue, 03 Sep 2019 10:00:57 +1000
Michael Ellerman <mpe@ellerman.id.au> wrote:

> Michal Such=C3=A1nek <msuchanek@suse.de> writes:
> > On Mon, 02 Sep 2019 12:03:12 +1000
> > Michael Ellerman <mpe@ellerman.id.au> wrote:
> > =20
> >> Michal Suchanek <msuchanek@suse.de> writes: =20
> >> > On bigendian ppc64 it is common to have 32bit legacy binaries but mu=
ch
> >> > less so on littleendian.   =20
> >>=20
> >> I think the toolchain people will tell you that there is no 32-bit
> >> little endian ABI defined at all, if anything works it's by accident. =
=20
> >
> > I have seen a piece of software that workarounds code issues on 64bit
> > by always compiling 32bit code. So it does work in some way. =20
>=20
> What software is that?

The only one I have seen is stockfish (v9)

>=20
> > Also it has been pointed out that you can still switch to BE even with
> > the 'fast-switch' removed. =20
>=20
> Yes we have a proper syscall for endian switching, sys_switch_endian(),
> which is definitely supported.
>=20
> But that *only* switches the endian-ness of the process, it does nothing
> to the syscall layer. So any process that switches to the other endian
> must endian flip syscall arguments (that aren't in registers), or flip
> back to the native endian before calling syscalls.

In other words just installing a chroot of binaries built for the other
endian won't work. You need something like qemu to do the syscall
translation or run full VM with a kernel that has the swapped endian
syscall ABI.

Thanks

Michal