Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
From:   Michael Ellerman <mpe@ellerman.id.au>
To:     Michal =?utf-8?Q?Such=C3=A1nek?= <msuchanek@suse.de>
Cc:     Michael Neuling <mikey@neuling.org>,
        Madhavan Srinivasan <maddy@linux.vnet.ibm.com>,
        Andrew Donnellan <andrew.donnellan@au1.ibm.com>,
        Arnd Bergmann <arnd@arndb.de>,
        David Hildenbrand <david@redhat.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Heiko Carstens <heiko.carstens@de.ibm.com>,
        linux-kernel@vger.kernel.org, Nicholas Piggin <npiggin@gmail.com>,
        "Eric W. Biederman" <ebiederm@xmission.com>,
        Diana Craciun <diana.craciun@nxp.com>,
        Paul Mackerras <paulus@samba.org>,
        Joel Stanley <joel@jms.id.au>,
        Allison Randal <allison@lohutok.net>,
        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>,
        Hari Bathini <hbathini@linux.ibm.com>,
        Alexander Viro <viro@zeniv.linux.org.uk>
Subject: Re: [PATCH v7 5/6] powerpc/64: Make COMPAT user-selectable disabled on littleendian by default.
In-Reply-To: <20190914122202.307707c0@naga>
References: <cover.1567198491.git.msuchanek@suse.de> <c7c88e88408588fa6fcf858a5ae503b5e2f4ec0b.1567198492.git.msuchanek@suse.de> <87ftlftpy7.fsf@mpe.ellerman.id.au> <20190902114239.32bd81f4@naga> <87h85us0xy.fsf@mpe.ellerman.id.au> <20190914122202.307707c0@naga>
Date:   Wed, 18 Sep 2019 15:36:24 +1000
Message-ID: <87k1a6w4h3.fsf@mpe.ellerman.id.au>
MIME-Version: 1.0
Content-Type: text/plain; charset=utf-8
Content-Transfer-Encoding: quoted-printable
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

Michal Such=C3=A1nek <msuchanek@suse.de> writes:
> 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=20
>> >> Michal Suchanek <msuchanek@suse.de> writes:=20=20
>> >> > On bigendian ppc64 it is common to have 32bit legacy binaries but m=
uch
>> >> > less so on littleendian.=20=20=20=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=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
>>=20
>> What software is that?
>
> The only one I have seen is stockfish (v9)

OK, not sure how many people are testing that on powerpc :)

>> > Also it has been pointed out that you can still switch to BE even with
>> > the 'fast-switch' removed.=20=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.

Yes that's right.

cheers