Re: [Xen-devel] 9p file system for xen

[Linda <lindaj@xxxxxxxx>]

On 11/16/2015 9:51 AM, Wei Liu wrote:
> On Mon, Nov 16, 2015 at 09:36:24AM -0700, Linda wrote:
> > Hi Wei,
> >
> > On 11/16/2015 8:16 AM, Wei Liu wrote:
> > > Hi Linda
> > >
> > > On Fri, Nov 13, 2015 at 10:23:22AM -0700, Linda wrote:
> > > > Hello,
> > > >       I worked this summer as an intern under Julien Grall and Wei Liu.  My
> > > > project was to develop a prototype/proof of concept xen front/back end for
> > > > the 9p file system.  I mostly hacked the virtio 9p system.
> > > >      This project was not complete, at the end of the summer.  Julien said
> > > > that you all wanted to include this in the next release of xen in January,
> > > > and offered to take it over.  I told Julien I wanted to continue working on
> > > > it, which I have been doing, very much in the background.
> > > >      I came upon a bug in my code recently that made me aware that I am not
> > > > clear what the expectation for what I deliver should be: i.e., whether it's
> > > > still a prototype, or whether this should be production software.
> > > >      Right now, I do not modify the toolstack (I never learned how), but
> > > > rather start and pause my guest, and then modify xenstore, manually.   I can
> > > > fix my bug in the same manner, but this will limit the usefulness of what I
> > > > deliver.  To do more will hit up against the limitations of my time and
> > > > knowledge.
> > > >      So please let me know what you're expecting, especially wrt the user
> > > > interface, and when I would need to complete everything for this release.
> > > If I interpret this correctly, you have a prototype that's working? Do
> > > you have your code somewhere?
> > No.  I hit a bug that I would fix differently, depending on my goal.
> > > I think we would still like to include it in next release if possible --
> > > that would require a properly implemented solution, not just a
> > > prototype.  Let's assess the current situation and then decide what to
> > The situation is, given my current knowledge and what my availability has
> > been (it may improve), I can either:
> >      a.  Get a decent prototype working by the end of the year.  This would
> > have certain values pre-written in xenstore, that I'm currently doing
> > manually.  There are potentially some issues with mounting that I suspect
> > need to be different for xen than they are for virtio - so either way, I
> > need a clarification of how xen people want this to work.
> >      b.  Make sure what I've written is working, and pass it on to someone
> > else to update the toolstack, and resolve the issues, described above.  In
> > this scenario, I would need to know how much time that someone would need
> > and just devote a week to getting this to them.
> Your description is too vague. I don't have clear idea what kind of bug
> you encountered and what suggestion I can give.
The bug is a timing issue:  During virtio's probe step, on the front 
end, it initialized the mount path.  Since at that time, the front end 
doesn't have access to the back end's entries in xenstore (AFIACT), I 
either need to put it in xenstore prior to starting, or move the access 
to this information to later in the initialization.
Note, I used the past tense on what virtio did, as of last summer: when 
I looked at it last week, it appears to have changed since I first used 
it as a template.    I need to investigate this further.
Finally, I've made no provision for how to mount more than one file 
system for the same guest.  This is a feature that virtio provides for 
in the front-end code (as do I), but I am unclear about how this works 
in the back-end or at the user level.  This is what I suspect will be 
different in xen, and I'd like some input on what it should look like.
> The code freeze for next release is going to be end of March next year.
> As software engineer often overestimates the progress he or she can
> make, I would say we shall aim for getting something working as soon as
> possible. Get the design straight and something clean by the end of this
> year would be good.
Sounds good to me.  I'm happy to keep working on this.  I just didn't 
want to find myself in a position where I needed to pass this on to 
someone else, but I didn't give that person enough time to finish what 
I'd done.
> > Either way, my next step is to sync up my qemu with the current qemu, and
> > merge everything, and then my github will be correct, at which point you'll
> > be able to access my most recent code.
> That would be a good first step. You don't actually need to fix the bug
> for that if you don't know how to proceed yet.
Good.  I'm glad we're on the same page, on this.

Linda
> Wei.

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