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

[Michael Ellerman]

Michal SuchÃnek <msuchanek@xxxxxxx> writes:
> On Tue, 03 Sep 2019 10:00:57 +1000
> Michael Ellerman <mpe@xxxxxxxxxxxxxx> wrote:
>> Michal SuchÃnek <msuchanek@xxxxxxx> writes:
>> > On Mon, 02 Sep 2019 12:03:12 +1000
>> > Michael Ellerman <mpe@xxxxxxxxxxxxxx> wrote:
>> >  
>> >> Michal Suchanek <msuchanek@xxxxxxx> 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