Version: current git HEAD (8edb622)

Good afternoon,

While experimenting with virtio-serial ports (using SystemTap's stapvirt
utility), we noticed something interesting. When stapvirt adds a
virtio-serial port, it simply adds the following XML

  <channel type='unix'>
    <source mode='bind' path='/my/path/to/sock.sock'/>
    <target type='virtio' name='org.systemtap.stapsh.0'/>
  </channel>

and relies on libvirt to get assigned to a virtio-serial controller
rather than creating an <address> element itself. So the resulting XML
might look like this:

  <channel type='unix'>
    <source mode='bind' path='/my/path/to/sock.sock'/>
    <target type='virtio' name='org.systemtap.stapsh.0'/>
    <address type='virtio-serial' controller='0' bus='0' port='1'/>
  </channel>

When creating another port, libvirt simply looks at previous
virtio-serial ports and adds one to the port number.

There are a few issues however:

1. libvirt will always use controller='0' and bus='0' even if there is
already a controller on a different index. This is not a big deal in
itself, since we can make a reasonable assumption that a controller
index 0 will exist before index 1, except that

2. libvirt does not respect the maximum number of ports available on a
controller. Even if the controller is declared with ports='4', libvirt
will gladly create e.g. 10 ports on that controller. This of course will
cause qemu to complain, e.g.:

error: Failed to start domain TestVM
error: internal error: early end of file from monitor: possible problem:
qemu-system-x86_64: -device
virtserialport,bus=virtio-serial0.0,nr=4,chardev=charchannel3,id=channel3,name=org.systemtap.stapsh.3:
virtio-serial-bus: Out-of-range port id specified, max. allowed: 3
qemu-system-x86_64: -device
virtserialport,bus=virtio-serial0.0,nr=4,chardev=charchannel3,id=channel3,name=org.systemtap.stapsh.3:
Device 'virtserialport' could not be initialized

This means that by default, libvirt will implicitly create a controller with index 0, then continuously add ports to that same controller well passed the limit of 31.

Expected:

libvirt respects the 'ports' attribute of the virtio-serial controller (assume 31 if missing) and creates a new controller if all the controllers are filled.

More generally, upon assigning an address, look at all the virtio-serial controllers available (including those to be inferred) and check if there are ports available on any of them. If not, then create a new controller (e.g. set controller to max_controller_index+1 and let it get implicitly created later during the definition process).


Thanks