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

[George Dunlap <george.dunlap@xxxxxxxxxx>]

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.

 -George

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