linux-ext4 - Re: [Qemu-devel] d_off field in struct dirent and 32-on-64 emulation

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Date:   Sun, 30 Dec 2018 13:59:17 +0000
From:   Peter Maydell <peter.maydell@...aro.org>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     Matthew Wilcox <willy@...radead.org>,
        Andreas Dilger <adilger@...ger.ca>,
        Florian Weimer <fw@...eb.enyo.de>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Linux API <linux-api@...r.kernel.org>,
        Ext4 Developers List <linux-ext4@...r.kernel.org>,
        Latchesar Ionkov <lucho@...kov.net>,
        libc-alpha <libc-alpha@...rceware.org>,
        Arnd Bergmann <arnd@...db.de>,
        Eric Van Hensbergen <ericvh@...il.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        lkml - Kernel Mailing List <linux-kernel@...r.kernel.org>,
        QEMU Developers <qemu-devel@...gnu.org>,
        Ron Minnich <rminnich@...dia.gov>,
        V9FS Developers <v9fs-developer@...ts.sourceforge.net>
Subject: Re: [Qemu-devel] d_off field in struct dirent and 32-on-64 emulation

On Sat, 29 Dec 2018 at 16:49, Andy Lutomirski <luto@...nel.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