Re: [Xen-devel] [PATCH] tools: specifically enable VirtFS in Linux QEMU builds

[Paul Durrant <Paul.Durrant@xxxxxxxxxx>]

> -----Original Message-----
> From: Stefano Stabellini [mailto:sstabellini@xxxxxxxxxx]
> Sent: 07 September 2018 17:58
> To: Paul Durrant <Paul.Durrant@xxxxxxxxxx>
> Cc: 'Stefano Stabellini' <sstabellini@xxxxxxxxxx>; xen-
> devel@xxxxxxxxxxxxxxxxxxxx; Andrew Cooper <Andrew.Cooper3@xxxxxxxxxx>;
> George Dunlap <George.Dunlap@xxxxxxxxxx>; Ian Jackson
> <Ian.Jackson@xxxxxxxxxx>; Jan Beulich <jbeulich@xxxxxxxx>; Julien Grall
> <julien.grall@xxxxxxx>; Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>;
> Tim (Xen.org) <tim@xxxxxxx>; Wei Liu <wei.liu2@xxxxxxxxxx>
> Subject: RE: [PATCH] tools: specifically enable VirtFS in Linux QEMU builds
> 
> On Fri, 7 Sep 2018, Paul Durrant wrote:
> > > -----Original Message-----
> > > From: Stefano Stabellini [mailto:sstabellini@xxxxxxxxxx]
> > > Sent: 06 September 2018 19:12
> > > To: Paul Durrant <Paul.Durrant@xxxxxxxxxx>
> > > Cc: xen-devel@xxxxxxxxxxxxxxxxxxxx; Andrew Cooper
> > > <Andrew.Cooper3@xxxxxxxxxx>; George Dunlap
> > > <George.Dunlap@xxxxxxxxxx>; Ian Jackson <Ian.Jackson@xxxxxxxxxx>; Jan
> > > Beulich <jbeulich@xxxxxxxx>; Julien Grall <julien.grall@xxxxxxx>;
> Konrad
> > > Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>; Stefano Stabellini
> > > <sstabellini@xxxxxxxxxx>; Tim (Xen.org) <tim@xxxxxxx>; Wei Liu
> > > <wei.liu2@xxxxxxxxxx>
> > > Subject: Re: [PATCH] tools: specifically enable VirtFS in Linux QEMU 
> > > builds
> > >
> > > On Thu, 6 Sep 2018, Paul Durrant wrote:
> > > > 9pfs support has been a documented feature since Xen 4.9, but QEMU
> will
> > > > not be built with backend support unless libcap and libattr dev packages
> > > > are installed.
> > > >
> > > > This patch modifies the README to call out those packages as pre-
> > > requisites
> > > > for Linux builds and specifically enables VirtFS in the configure line
> > > > for QEMU so that an error message is displayed if they are missing.
> > > >
> > > > Signed-off-by: Paul Durrant <paul.durrant@xxxxxxxxxx>
> > >
> > > Thank you, Paul!
> >
> > NP. I'm at the point of seriously considering a Windows frontend, which is
> why I've been playing.
> >
> > BTW, as a heads-up... Before I realised QEMU was not building the
> backend, I tried a modprobe of the Xen 9pfs transport module in a Linux
> guest and immediately hit a kernel BUG (NULL ptr IIRC) which was apparently
> down to the frontend expecting the backend to be present when its probe
> routine is run, rather than deferring things until the backend goes into
> InitWait (which is what a frontend generally should do). Also, it's 
> non-obvious
> why I have to explicitly modprobe anyway... Shouldn't the module get
> demand-loaded when I run mount -t 9pfs blah?
> 
> Interesting. I don't recall ever testing this is a scenario, so I am not
> surprised it is broken. I don't know why the module doesn't get loaded
> automatically. FYI another thing missing is suspend/resume support in
> the driver.

I see. I guess that probably means cycling through closing->closed and back to 
initwait has not been exercised in the backend. I'll bear that in mind when I 
get to that stage. Thanks :-)

  Paul

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