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

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

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.

Roger.

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