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

From: Michal SuchÃnek
Date: Sat Sep 14 2019 - 06:22:17 EST

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)

> > 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.