In a Xen environment, programs using libvir have to execute in "Domain 0",
which is the primary Linux OS loaded on the machine. That OS kernel provides
most if not all of the actual drivers used by the set of domains. It also
runs the Xen Store, a database of informations shared by the hypervisor, the
kernels, the drivers and the xen daemon. Xend, the xen daemon, also runs in
Domain0 and supervise the control and execution of the sets of domains. The
hypervisor, drivers, kernels and daemons communicate though a shared system
bus implemented in the hypervisor. The figure below tries to provide a view
of this environment:  The library can be initialized in 2 ways depending on the level of
priviledge of the embedding program. If it runs with root access,
virConnectOpen() can be used, it will use a read/write connection to the Xen
Store and use Xen Hypervisor calls when possible. If it runs without root
access virConnectOpenReadOnly() should be used, it will try to open the
read-only socket /var/run/xenstored/socket_ro to connect to the
Xen Store. In this case use of hypervisor calls and write to the Xen Store
will not be possible, restraining the amount of APIs available and slowing
down information gathering about domains. We expect libvir will also conmnect
to the Xend daemon for some of the domain control operations, though this is
not the case in the 0.0.1 initial version. Daniel Veillard |
