A channel is a low-bandwidth private byte stream similar to a serial link. Typical uses of channels are
Channels are similar to virtio-serial devices and emulated serial links. Channels are intended to be used in the implementation of libvirt
Note: if an application requires a high-bandwidth link then it should use vchan instead.
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:
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)
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/
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
The in-guest agent receives the configuration and applies it.
Using channels avoids having to use a temporary disk device or network connection.
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.
Hotplug and unplug of channels is not currently implemented.
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 <email@example.com> D: Share clipboard data via an in-guest agent. See: http://wiki.xenproject.org/wiki/Clipboard_sharing_protocol