libvirt users - Mar 2019 - Problem passing qemu cmdline option when using <qemu:commandline> in domain xml.

【Sorry there I maked some mistake in my first email, here is the correct one.】
I am trying to pass-through the parallel port of the host to the guest.
When I just used qemu-system-x86_64 with option '-chardev
parallel,id=charparallel0,path=/dev/parport0 -device
isa-parallel,chardev=charparallel0,id=parallel0', it worked perfect.
But when I tried to pass these option by adding lines below to the domain xml,
...
</device>
<qemu:commandline>
  <qemu:args value='-chardev'/>
  <qemu:args
value='parallel,id=charparallel0,path=/dev/parport0'/>
  <qemu:args value='-device'/>
  <qemu:args
value='isa-parallel,chardev=charparallel0,id=parallel0'/>
</qemu:commandline>
...
it failed and showed:
[root@localhost local]# virsh start test02
error: Failed to start domain test02
error: internal error: process exited while connecting to monitor: 2019-03-27
02:28:12.507+0000: Domain id=32 is  tainted: custom-argv
2019-03-27T02:28:12.599326Z qemu-system-x86_64: -chardev
parallel,id=charparallel0,path=/dev/parport0: Could not  open
'/dev/parport0': No such file or directory


I dont know why it showed a 'No such file or directory'. The
/dev/parport0 is actually here. I have tried setting chmod 666 to /dev/parport0
and shutdown the selinux, but it always show the same thing.
Please give me some help, thanks.

On Wed, Mar 27, 2019 at 03:05:17 +0800, Chenzijian
wrote:
> 【Sorry there I maked some mistake in my first email, here is the correct
one.】
> I am trying to pass-through the parallel port of the host to the guest.
> When I just used qemu-system-x86_64 with option '-chardev
parallel,id=charparallel0,path=/dev/parport0 -device
isa-parallel,chardev=charparallel0,id=parallel0', it worked perfect.
> But when I tried to pass these option by adding lines below to the domain
xml,
> ...
> </device>
> <qemu:commandline>
>   <qemu:args value='-chardev'/>
>   <qemu:args
value='parallel,id=charparallel0,path=/dev/parport0'/>
>   <qemu:args value='-device'/>
>   <qemu:args
value='isa-parallel,chardev=charparallel0,id=parallel0'/>
> </qemu:commandline>
Note that using this should not be required as libvirt does support
<parallel> devices.

> ...
> it failed and showed:
> [root@localhost local]# virsh start test02
> error: Failed to start domain test02
> error: internal error: process exited while connecting to monitor:
2019-03-27 02:28:12.507+0000: Domain id=32 is  tainted: custom-argv
> 2019-03-27T02:28:12.599326Z qemu-system-x86_64: -chardev
parallel,id=charparallel0,path=/dev/parport0: Could not  open
'/dev/parport0': No such file or directory
> 
> 
> I dont know why it showed a 'No such file or directory'. The
/dev/parport0 is actually here. I have tried setting chmod 666 to /dev/parport0
and shutdown the selinux, but it always show the same thing.
> Please give me some help, thanks.
libvirt runs qemu in a "container" where /dev/is not fully populated,
but rather only necessary devices are present.

For the example above you should be able to use the <parallel> element
directly.

You can work around the containerization of /dev/ by adding the correct
device to cgroup_device_acl in /etc/libvirt/qemu.conf