Re: [Qemu-devel] d_off field in struct dirent and 32-on-64 emulation

[Peter Maydell <peter.maydell@linaro.org>]

On Sat, 29 Dec 2018 at 16:49, Andy Lutomirski <luto@kernel.org> wrote:
> > Could you use a prctl to set whether you were running in 32 or 64 bit
> > mode?  Or do you change which kind of task you're emulating too often
> > to make this a good idea?

QEMU's linux-user mode always only runs the single process,
which is a fixed guest architecture. But it also wants to
make system calls on its own behalf, as well as the ones it
is passing through from the guest, and I suspect it would
confuse the host libc if we changed the semantics of those
under its feet.

> How would this work?  We already have the separate
> COMPAT_DEFINE_SYSCALL entries *and* in_compat_syscall(). Now we’d have
> a third degree of freedom.
>
> Either the arches people care about should add reasonable ways to
> issue 32-bit syscalls from 64-bit mode or there should be an explicit
> way to ask for the 32-bit directory offsets.

The first of those is not sufficient for QEMU if done
as a per-architecture thing, because there may not even be
a 32-bit syscall interface on the host kernel. The second
sounds better -- there's nothing conceptually architecture
specific about what we want to do or which is tied to the
idea of whether there's a 32-bit compat mode in the host
architecture or not.

thanks
-- PMM