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[libvirt] Using vmchannel?

I was just looking at the new vmchannel feature in QEMU and was
wondering if we could make this available in libvirt.

Firstly, since there isn't much documentation, I should explain how
vmchannel works:

  [1] You pass an extra parameter on the qemu command line:

    qemu [...] -vmchannel <port>:<dev>

  where <port> is the TCP port number (see below) and <dev> is
  a standard qemu device description.  As an example:

    qemu [...] -vmchannel 600:unix:/some/path

  [2] A new character device appears in the host, eg. Unix domain
    socket called "/some/path".

  [3] In the host, a userspace program should open a socket, bind(2)
    it to /some/path and listen(2) and accept(2) on it.

  [4] In the guest, any process may connect(2) to TCP
    (or whatever port was selected).  Each connection in the guest
    causes the listener in the host to accept(2).

  [5] This is only designed for low-bandwidth, low-performance

  [6] Multiple vmchannels are supported.

  [7] Host cannot initiate a connection.

My plan to implement this would be to add a new network interface type
to the domain XML:

  <interface type='vmchannel'>
    <source port='600'/>
    <target path='/some/path'/>

Only Unix domain socket paths would be allowed on the host side, and
the path would be expected to exist already with suitable permissions.

Note that I think this would also allow guests to communicate with the
libvirtd on the host (not by default, of course, but if users wanted
to configure it that way), for example:

  <interface type='vmchannel'>
    <source port='600'/>
    <target path='/var/run/libvirt/libvirt-sock'/>

One problem is that it is qemu/kvm-only.  It is built on top of
virtio, and virtio is meant to become a standard driver subsystem for
all full virtualization systems.



Richard Jones, Emerging Technologies, Red Hat  http://et.redhat.com/~rjones
virt-p2v converts physical machines to virtual machines.  Boot with a
live CD or over the network (PXE) and turn machines into Xen guests.

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