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

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Fri, Sep 07, 2018 at 08:35:11AM +0000, 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?
> 
> > Do we need to do anything for the configure stuff
> > (AC_CHECK_LIB in tools/configure.ac)?
> 
> That's certainly a reasonable belt'n'braces approach so that folks don't have 
> to wait until their tools build fails to find out what they need. I'll have a 
> look at that.

IMO I don't think we should be duplicating the checks that QEMU
configure already does in our configure script. The best option would
be to run whatever external configure scripts there are together with
our own configure script, but this is quite complicated because the
qemu-dir might have to be fetched at the point where configure runs.

Roger.

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