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

From: Theodore Y. Ts'o
Date: Fri Dec 28 2018 - 23:18:52 EST

On Sat, Dec 29, 2018 at 03:37:21AM +0100, Dominique Martinet wrote:
> > Are there going to be cases where a process or a thread will sometimes
> > want the 64-bit interface, and sometimes want the 32-bit interface?
> > Or is it always going to be one or the other? I wonder if we could
> > simply add a new flag to the process personality(2) flags.
> That would likely work for qemu user, but the qemu system+9p case is
> going to be more painful..
> More precisely, the 9p protocol does not plan for anything other than
> 64bit offset so if the vfs needs to hand out a 32bit offset we'll need
> to make a correspondance table between the 32bit offsets we hand off and
> the 64bit ones to use; unless some flag can be passed at lopen to tell
> the server to always hand out 32bit offsets for this directory... And if
> we do that then 9p servers will need a way to use both APIs in parallel
> for both types of directories.

How about if we add a fcntl(2) mediated flag, which is tied to a
struct file? Would that be more or less painful for 9p and qemu

- Ted