libvir architecture

This is Xen specific since this is the only hypervisor supported at the moment

When running 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 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 three different ways to connect to the Xen infrastructure:

• a connection to the Xen Daemon though an HTTP RPC layer
• a read/write connection to the Xen Store
• use Xen Hypervisor calls

The library will usually interract with the Xen daemon for any operation changing the state of the system, but for performance and accuracy reasons may talk directly to the hypervisor when gathering state informations at least when possible (i.e. when the running program using libvir has root priviledge access).

If it runs without root access virConnectOpenReadOnly() should be used to connect to initialize the library. It will try to open the read-only socket /var/run/xenstored/socket_ro to connect to the Xen Store and also try to use the RPC to the Xen daemon. 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.