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

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Date:   Wed, 18 Sep 2019 15:36:24 +1000
From:   Michael Ellerman <mpe@...erman.id.au>
To:     Michal Suchánek <msuchanek@...e.de>
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.

Michal Suchánek <msuchanek@...e.de> writes:
> 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)

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.  
>> 
>> 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.

Yes that's right.

cheers