Print-friendly Version

Sun xVM Hypervisor Overview

Penny Cotten, May 2008

This Sun xVM Hypervisor Overview discusses the Sun xVM hypervisor software on the x86 family of processor architectures, and virtual machine instances known as domains. The hypervisor is based on the work of the Xen community.

About Virtualization

Virtualization provides a way to bypass interoperability constraints. The goal of virtualization is to move from managing individual datacenter components to managing pools of resources. Successful server virtualization can lead to improved server utilization and more efficient use of server assets. Server virtualization is also important for successful server consolidation projects that maintain the isolation of separate systems.

Virtualization is driven by the need to consolidate multiple hosts and services on a single machine. Virtualization reduces costs through the sharing of hardware, infrastructure, and administration.

The Sun xVM Hypervisor and the Control Domain

This section discusses the roles of the hypervisor and domain 0, the control domain. The hypervisor is based on the open source Xen project.

The hypervisor is a virtualization system for running multiple operating system instances simultaneously on a single x86 compatible machine.

Each virtual machine instance is called a domain. There are two kinds of domains: the control domain and the guest domain. The control domain is also known as domain 0, or dom0. A guest operating system, or unprivileged domain, is also called a domain U or domU. Each virtual machine guest domain runs a full instance of an operating system.

Supported virtual machine configurations include a control domain (domain 0), and any of the following guests:

• Solaris guest domain

• Linux guest domain

• FreeBSD guest domain

• Windows guest domain

How Hypervisors Work

A hypervisor partitions a single physical machine into multiple virtual machines, to provide server consolidation and utility computing. Existing applications and binaries run unmodified.

The hypervisor presents a virtual machine to virtual machine guests. The hypervisor forms a layer between the software running on the virtual machine and the hardware. This separation enables the hypervisor to control how guest operating systems that are running inside a virtual machine use hardware resources. The hypervisor provides a uniform view of underlying hardware.

Hypervisor and Domain Interaction

The hypervisor is responsible for controlling and executing each of the domains. The control tools for managing the domains run under the specialized control domain, domain 0 (dom0).

The hypervisor virtualizes the system's hardware. The hypervisor transparently shares and partitions the system's resources, such as CPUs, memory, and network interface cards (NICs), among the user domains. The hypervisor performs the low-level work required to provide a virtualized platform for operating systems.

The hypervisor relies primarily on the control domain to provide the following:

• Which guest domains are created

• Which resources the guest domains can access

• How much memory a given guest domain can have

• Which devices a guest domain can access

  The hypervisor delegates control of the machine's physical devices to the control domain, domain 0.

Only the control domain has access to physical devices. The guest domains running on the host are presented with virtualized devices. The domains interact with the virtualized devices in the same way that the domains would interact with the physical devices.

Supported Virtualization Modes

There are two basic types of hypervisor virtualization: full virtualization and paravirtualization. The hypervisor supports both modes.

Full virtualization allows any x86 operating system, including Solaris, Linux, and Windows systems, to run in a guest domain. A fully virtualized guest domain is referred to as a hardware-assisted virtual machine (HVM). An HVM guest domain runs an unmodified operating system. Fully virtualized guest domains are supported under xVM with virtualization extensions available on Intel-VT and AMD-V processors. These extensions must be present and enabled. Some BIOS versions disable the extensions by default.

Paravirtualization requires changes to the operating system. Only specific operating systems can be hosted in a paravirtualized guest domain. Currently these systems are limited to Solaris, Linux, and FreeBSD.

A system can have both paravirtualized and fully virtualized domains running simultaneously.

For paravirtualized mode and for all types of operating systems, the only requirement is that the operating system be modified to support the virtual device interfaces.

Because the control domain must work closely with the hypervisor layer, the control domain is always paravirtualized.

Related Information

For more information about the Sun xVM hypervisor and virtual machines, see:

• OpenSolaris Xen Community

• System Administration Guide: Virtualization Using the Solaris Operating System

Unless otherwise licensed, code in all technical manuals herein (including articles, FAQs, samples) is provided under this License.