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

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

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.

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