NAME

       xen-pv-channel - Xen PV Channels

DESCRIPTION

       A channel is a low-bandwidth private byte stream similar to a serial link. Typical uses of channels are

       1.  to provide initial configuration information to a VM on boot (example use: CloudStack's cloud-early-
           config service)

       2.  to signal/query an in-guest agent (example use: oVirt's guest agent)

       Channels are similar to virtio-serial devices and emulated serial links.  Channels are intended to be
       used in the implementation of libvirt s when running on Xen.

       Note: if an application requires a high-bandwidth link then it should use vchan instead.

   How to use channels: an example
       Consider a cloud deployment where VMs are cloned from pre-made templates, and customised on first boot by
       an in-guest agent which sets the IP address, hostname, ssh keys etc. To install the system the cloud
       administrator would first:

       1.  Install a guest as normal (no channel configuration necessary)

       2.  Install the in-guest agent specific to the cloud software. This will prepare the guest to communicate
           over the channel, and also prepare the guest to be cloned safely (sometimes known as "sysprepping")

       3.  Shutdown the guest

       4.  Register the guest as a template with the cloud orchestration software

       5.  Install the cloud orchestration agent in dom0

       At runtime, when a cloud tenant requests that a VM is created from the template, the sequence of events
       would be: (assuming a Linux domU)

       1.  A VM is "cloned" from the template

       2.  A unique Unix domain socket path in dom0 is allocated (e.g. /my/cloud/software/talk/to/domain/)

       3.  Domain configuration is created for the VM, listing the channel name expected by the in-guest agent.
           In xl syntax this would be:

           channel = [ "connection=socket, name=org.my.cloud.software.agent.version1, path =
           /my/cloud/software/talk/to/domain/" ]

       4.  The VM is started

       5.  In dom0 the cloud orchestration agent connects to the Unix domain socket, writes a handshake message
           and waits for a reply

       6.  Assuming the guest kernel has CONFIG_HVC_XEN_FRONTEND set then the console driver will generate a
           hotplug event

       7.  A udev rule is activated by the hotplug event.

           The udev rule would look something like:

           SUBSYSTEM=="xen", DEVPATH=="/devices/console-[0-9]", RUN+="xen-console-setup"

           where the "xen-console-setup" script would read the channel name and make a symlink in
           /dev/xen-channel/org.my.cloud.software.agent.version1 pointing to /dev/hvcN. N is the same number as
           the number in "/devices/console-[0-9]".  In other words, "/devices/console-2" maps to /dev/hvc2.

       8.  The in-guest agent uses inotify to see the creation of the /dev/xen-channel symlink and opens the
           device.

       9.  The in-guest agent completes the handshake with the dom0 agent

       10. The dom0 agent transmits the unique VM configuration: hostname, IP address, ssh keys etc etc

       11. The in-guest agent receives the configuration and applies it.

       Using channels avoids having to use a temporary disk device or network connection.

   Design recommendations and pitfalls
       It's necessary to install channel-specific software (an "agent") into the guest before you can use a
       channel. By default a channel will appear as a device which could be mistaken for a serial port or
       regular console. It is known that some software will proactively seek out serial ports and issue AT
       commands at them; make sure such software is disabled!

       Since channels are identified by names, application authors must ensure their channel names are unique to
       avoid clashes. We recommend that channel names include parts unique to the application such as a domain
       names. To assist prevent clashes we recommend authors add their names to our global channel registry at
       the end of this document.

   Limitations
       Hotplug and unplug of channels is not currently implemented.

   Channel name registry
       It is important that channel names are globally unique. To help ensure that no-one's name clashes with
       yours, please add yours to this list.

           Key:
           N: Name
           C: Contact
           D: Short description of use, possibly including a URL to your software or API

           N: org.xenproject.guest.clipboard.0.1
           C: David Scott <dave.scott@citrix.com>
           D: Share clipboard data via an in-guest agent. See:
              https://wiki.xenproject.org/wiki/Clipboard_sharing_protocol