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

[Paul Durrant <Paul.Durrant@xxxxxxxxxx>]

> -----Original Message-----
> From: George Dunlap [mailto:george.dunlap@xxxxxxxxxx]
> Sent: 07 September 2018 16:26
> To: Roger Pau Monne <roger.pau@xxxxxxxxxx>; Paul Durrant
> <Paul.Durrant@xxxxxxxxxx>
> Cc: 'Stefano Stabellini' <sstabellini@xxxxxxxxxx>; Wei Liu
> <wei.liu2@xxxxxxxxxx>; Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>;
> Andrew Cooper <Andrew.Cooper3@xxxxxxxxxx>; Tim (Xen.org)
> <tim@xxxxxxx>; Julien Grall <julien.grall@xxxxxxx>; Jan Beulich
> <jbeulich@xxxxxxxx>; Ian Jackson <Ian.Jackson@xxxxxxxxxx>; xen-
> devel@xxxxxxxxxxxxxxxxxxxx
> Subject: Re: [Xen-devel] [PATCH] tools: specifically enable VirtFS in Linux
> QEMU builds
> 
> On 09/07/2018 03:57 PM, Roger Pau Monné wrote:
> > 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.
> 
> Not the least because the dependencies may change.  I think adding an
> "--enable-9pfs" option which will pass on the requisite "--enable" to
> qemu (such that qemu build will fail if the prereqs are not present)
> makes the most sense.

I'd be happy with that if others are.

  Paul

> 
>  -George
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