VMchannel Requirements

Requirements

• We want an interface between the guest and the host

• The channel is to be used for simple communication, like sharing of the clipboard between the user desktop and the guest desktop
  • For relatively low rate of data transfer -- a few MB/s
  • Events to be delivered to the guest, like 'shutdown', 'reboot', 'logoff'
  • Queries to the guest, like 'which users are logged in'

• Survive live migration

• Support for multiple agents (consumers of the data) on the guest

• Multiple channels could be opened at the same time

• In multi-channels case, one blocked channel shouldn't block communication between others (or one channel shouldn't hog all the bandwidth)

• Stable ABI (for future upgrades)

• Channel addressing
  • An agent in the guest should be able to find the channel it's interested in

• Dynamic channel creation

• Security
  • No threats to the host
  • Unprivileged user should be able to use the channel

• Should work out of the box, without any configuration necessary on the part of the guest

• Notifications of channels being added / removed (hotplugging)

• An API inside qemu to communicate with agents in the guest

History

A few reasons why the obvious solutions do not work:

• via the fully emulated serial device.
  • performance (exit per byte)
  • scalability - only 4 serial ports per guest
  • accessed by root only in the guest

• via TCP/IP network sockets
  • The guest may not have networking enabled
  • The guest firewall may block access to the host IPs
  • Windows can't bind sockets to specific ethernet interfaces

• via user net
  http://thread.gmane.org/gmane.comp.emulators.qemu/35780

• via slirp
  This implementation does exist upstream as "-net channel" http://www.nabble.com/-PATCH--specify-vmchannel-as-a-net-option-td21911523.html
  • Again, based on networking so same drawbacks mentioned above apply
  • Currently used by libguestfs

Use Cases

• Guest - Host clipboard copy/paste operations
  • By a VMM or via an internal API within qemu
• libguestfs (offline usage)
  • For poking inside a guest to fetch the list of installed apps, etc.
• Online usage
  • Locking desktop session when vnc session is closed