Oracle Solaris 10 as an x86 Guest - Oracle Solaris 10 System Virtualization Essentials - page 115

as 64 GB. With a 64-bit hypervisor, it is possible to access even more than 64 GB of
memory and, therefore, to support a greater number of virtual machines.

4.1.3 Xen Open-Source Project-Based Hypervisors
The following technologies are derived from the work of the open-source Xen project:

Oracle VM Server for x86

■

xVM hypervisor

■

Citrix XenServer

■

For further details on the Xen project, see http://www.xen.org .
Xen is open-source software that was released under the terms of the GNU
General Public License. Its hypervisor uses a combination of paravirtualization
and hardware-assisted virtualization. Paravirtualization allows for collaboration
between the operating environment and the virtualization platform, which in turn
enables the creation of a simple and lightweight hypervisor. For guest operating
environments that are not fully paravirtualized, the Xen-based hypervisor uses
hardware virtualization assist technologies.
Two types of domains are supported: the privileged domain 0 (also referred to
as dom0) and unprivileged guest domains (referred to as domU).
While the hypervisor performs low-level virtualization functionality, it relies on
dom0 for the following functions:

Creating guest domains

■

Controlling access to system resources by the guest domains

■

Controlling the amount of memory allocated to a guest domain

■

Determining which real physical devices a guest domain can access

■

Because Xen-based hypervisors do not include any device drivers, the hyper-
visor delegates control of the machine's physical devices to dom0. That domain
must work closely with the hypervisor layer; therefore, dom0 is always paravir-
tualized. The hypervisor boots before dom0 and then boots dom0 as its first guest
domain. Thus this domain is, in effect, a special-purpose paravirtualized guest
that provides the control of the virtualization functionality.
A guest domain can be either paravirtualized (PV) or a hardware virtual ma-
chine (HVM).
In paravirtualization , the guest operating environment is aware that it is run-
ning on top of a hypervisor and has additional features that can expedite device
access and support the concept of stable devices. Stable devices free the guest from