linux-kernel - Re: [PATCH v7 5/6] powerpc/64: Make COMPAT user-selectable disabled on littleendian by default.

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Date:   Sat, 14 Sep 2019 12:22:02 +0200
From:   Michal Suchánek <msuchanek@...e.de>
To:     Michael Ellerman <mpe@...erman.id.au>
Cc:     Michael Neuling <mikey@...ling.org>,
        Madhavan Srinivasan <maddy@...ux.vnet.ibm.com>,
        Andrew Donnellan <andrew.donnellan@....ibm.com>,
        Arnd Bergmann <arnd@...db.de>,
        David Hildenbrand <david@...hat.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Heiko Carstens <heiko.carstens@...ibm.com>,
        linux-kernel@...r.kernel.org, Nicholas Piggin <npiggin@...il.com>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Diana Craciun <diana.craciun@....com>,
        Paul Mackerras <paulus@...ba.org>,
        Joel Stanley <joel@....id.au>,
        Allison Randal <allison@...utok.net>,
        Breno Leitao <leitao@...ian.org>,
        Firoz Khan <firoz.khan@...aro.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        linuxppc-dev@...ts.ozlabs.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Hari Bathini <hbathini@...ux.ibm.com>,
        Alexander Viro <viro@...iv.linux.org.uk>
Subject: Re: [PATCH v7 5/6] powerpc/64: Make COMPAT user-selectable disabled
 on littleendian by default.

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

> Michal Suchánek <msuchanek@...e.de> writes:
> > On Mon, 02 Sep 2019 12:03:12 +1000
> > Michael Ellerman <mpe@...erman.id.au> wrote:
> >  
> >> Michal Suchanek <msuchanek@...e.de> writes:  
> >> > On bigendian ppc64 it is common to have 32bit legacy binaries but much
> >> > less so on littleendian.    
> >> 
> >> 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.  
> >
> > 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.  
> 
> What software is that?

The only one I have seen is stockfish (v9)

> 
> > Also it has been pointed out that you can still switch to BE even with
> > the 'fast-switch' removed.  
> 
> Yes we have a proper syscall for endian switching, sys_switch_endian(),
> which is definitely supported.
> 
> 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