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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
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