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

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

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 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.

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