Re: [Xen-devel] Commit 3a6c9 breaks QEMU on FreeBSD/Xen

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Tue, 24 Jan 2017, Stefano Stabellini wrote:
> On Tue, 24 Jan 2017, Roger Pau Monné wrote:
> > Hello,
> > 
> > The following commit:
> > 
> > commit 3a6c9172ac5951e6dac2b3f6cbce3cfccdec5894
> > Author: Juergen Gross <jgross@xxxxxxxx>
> > Date:   Tue Nov 22 07:10:58 2016 +0100
> > 
> > xen: create qdev for each backend device
> > 
> > Prevents me from running QEMU on FreeBSD/Xen, the following is printed on 
> > the
> > QEMU log:
> > 
> > char device redirected to /dev/pts/2 (label serial0)
> > xen be core: xen be core: can't open gnttab device
> > can't open gnttab device
> > xen be core: xen be core: can't open gnttab device
> > can't open gnttab device
> > 
> > # xl create -c ~/domain.cfg
> > Parsing config from /root/domain.cfg
> > libxl: error: libxl_dm.c:2201:device_model_spawn_outcome: Domain 32:domain 
> > 32 device model: spawn failed (rc=-3)
> > libxl: error: libxl_create.c:1506:domcreate_devmodel_started: Domain 
> > 32:device model did not start: -3
> > libxl: error: libxl_dm.c:2315:kill_device_model: Device Model already exited
> > libxl: error: libxl.c:1572:libxl__destroy_domid: Domain 32:Non-existant 
> > domain
> > libxl: error: libxl.c:1531:domain_destroy_callback: Domain 32:Unable to 
> > destroy guest
> > libxl: error: libxl.c:1458:domain_destroy_cb: Domain 32:Destruction of 
> > domain failed
> > # cat /var/log/xen/qemu-dm-domain.log
> > char device redirected to /dev/pts/2 (label serial0)
> > xen be core: xen be core: can't open gnttab device
> > can't open gnttab device
> > xen be core: xen be core: can't open gnttab device
> > can't open gnttab device
> > 
> > I'm not really familiar with any of that code, but I think that using
> > qdev_init_nofail is wrong, since on FreeBSD/Xen for example we don't yet
> > support the gnttab device, so initialization of the Xen Qdisk backend can 
> > fail
> > (and possibly the same applies to Linux if someone decides to compile a 
> > kernel
> > without the gnttab device). Yet QEMU can be used without the Qdisk backend.
> 
> How did you manage to configure QEMU before? The configure script had
> xc_gnttab_open calls in it up to Xen 4.6.

I know the answer! Because the configure script only compiles the code,
doesn't try to run it. xc_gnttab_open compiled correctly but returned
error when executed. Is that right?


> I am happy to support a use case where the kernel doesn't have gntdev,
> but it needs to be explicit: we need to detect it in the configure
> script, then avoid the initialization of devices which require it.

I would still prefer configure to be able to detect this case. If it
cannot be made to detect it, then we can try to figure out a way to
catch the initialization errors at run time.

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