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Re: [V9fs-developer] [Qemu-devel] d_off field in struct dirent and 32-on-64 emulation
- From: Dominique Martinet <asmadeus at codewreck dot org>
- To: "Theodore Y. Ts'o" <tytso at mit dot edu>, Peter Maydell <peter dot maydell at linaro dot org>, Andreas Dilger <adilger at dilger dot ca>, Florian Weimer <fw at deneb dot enyo dot de>, linux-fsdevel <linux-fsdevel at vger dot kernel dot org>, Linux API <linux-api at vger dot kernel dot org>, Ext4 Developers List <linux-ext4 at vger dot kernel dot org>, lucho at ionkov dot net, libc-alpha at sourceware dot org, Arnd Bergmann <arnd at arndb dot de>, ericvh at gmail dot com, hpa at zytor dot com, lkml - Kernel Mailing List <linux-kernel at vger dot kernel dot org>, QEMU Developers <qemu-devel at nongnu dot org>, rminnich at sandia dot gov, v9fs-developer at lists dot sourceforge dot net
- Date: Sat, 29 Dec 2018 05:04:13 +0100
- Subject: Re: [V9fs-developer] [Qemu-devel] d_off field in struct dirent and 32-on-64 emulation
- References: <87bm56vqg4.fsf@mid.deneb.enyo.de> <CAFEAcA92m4vhzjJ+B=mP_o6Wfhx1XSKo3uWxah3osh=u5UXFuw@mail.gmail.com> <9C6A7D45-CF53-4C61-B5DD-12CA0D419972@dilger.ca> <CAFEAcA9W+JK7_TrtTnL1P2ES1knNPJX9wcUvhfLwxLq9augq1w@mail.gmail.com> <20181229021157.GG5864@mit.edu> <20181229023721.GA9291@nautica> <20181229031416.GH5864@mit.edu>
Theodore Y. Ts'o wrote on Fri, Dec 28, 2018:
> 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
> system+9p?
Hmm. 9P2000.L doesn't have anything akin to fcntl either, the only two
obvious places where we could pass a flag is lopen (which already
handles a bunch of linux-specific flags, e.g. passing O_LARGEFILE
O_NOATIME etc will just forward these through for qemu/diod at least),
or adding a new parameter to the 9p readdir.
The former would let us get away without modifying the protocol as
servers will just ignore flags they don't handle on implementations I
checked, so it'd definitely be the least effort choice from what I can
tell.
On the other hand a fcntl would solve the server-side problem, it'd
allow the server to request appropriately-sized offsets per fd, so it's
a good start; we "just" need to figure how to translate that on the wire.
--
Dominique Martinet | Asmadeus