Xen

 Xen (pronounced /ˈzɛn/) is a type-1 hypervisor, providing services that allow multiple computer operating systems to execute on the same computer hardware concurrently. It was originally developed by the University of Cambridge Computer Laboratory and is now being developed by the Linux Foundation with support from Intel, Citrix, Arm Ltd, Huawei, AWS, Alibaba Cloud, AMD, Bitdefender and epam.

The Xen Project community develops and maintains Xen Project as free and open-source software, subject to the requirements of the GNU General Public License (GPL), version 2. Xen Project is currently available for the IA-32, x86-64 and ARM instruction sets.^[4]

Software architecture[edit]

Xen Project runs in a more privileged CPU state than any other software on the machine, except for Firmware.

Responsibilities of the hypervisor include memory management and CPU scheduling of all virtual machines ("domains"), and for launching the most privileged domain ("dom0") - the only virtual machine which by default has direct access to hardware. From the dom0 the hypervisor can be managed and unprivileged domains ("domU") can be launched.^[5]

The dom0 domain is typically a version of Linux or BSD. User domains may either be traditional operating systems, such as Microsoft Windows under which privileged instructions are provided by hardware virtualization instructions (if the host processor supports x86 virtualization, e.g., Intel VT-x and AMD-V),^[6] or paravirtualized operating systems whereby the operating system is aware that it is running inside a virtual machine, and so makes hypercalls directly, rather than issuing privileged instructions.

Xen Project boots from a bootloader such as GNU GRUB, and then usually loads a paravirtualized host operating system into the host domain (dom0).

History[edit]

Xen originated as a research project at the University of Cambridge led by Ian Pratt, a senior lecturer in the Computer Laboratory, and his PhD student Keir Fraser. The first public release of Xen was made in 2003, with v1.0 following in 2004. Soon after, Pratt and Fraser along with other Cambridge alumni including Simon Crosby and founding CEO Nick Gault created XenSource Inc. to turn Xen into a competitive enterprise product.

To support embedded systems such as smartphone/ IoT with relatively scarce hardware computing resources, the Secure Xen ARM architecture on an ARM CPU was exhibited at Xen Summit on April 17, 2007, held in IBM TJ Watson.^[7][8] The first public release of Secure Xen ARM source code was made at Xen Summit on June 24, 2008^[9][10] by Sang-bum Suh,^[11] a Cambridge alumnus, in Samsung Electronics.

On October 22, 2007, Citrix Systems completed its acquisition of XenSource,^[12] and the Xen Project moved to the xen.org domain. This move had started some time previously, and made public the existence of the Xen Project Advisory Board (Xen AB), which had members from Citrix, IBM, Intel, Hewlett-Packard, Novell, Red Hat, Sun Microsystems and Oracle. The Xen Advisory Board advises the Xen Project leader and is responsible for the Xen trademark,^[13] which Citrix has freely licensed to all vendors and projects that implement the Xen hypervisor.^[14] Citrix also used the Xen brand itself for some proprietary products unrelated to Xen, including XenApp and XenDesktop.

On April 15, 2013, it was announced that the Xen Project was moved under the auspices of the Linux Foundation as a Collaborative Project.^[15] The Linux Foundation launched a new trademark for "Xen Project" to differentiate the project from any commercial use of the older "Xen" trademark. A new community website was launched at xenproject.org^[16] as part of the transfer. Project members at the time of the announcement included: Amazon, AMD, Bromium, CA Technologies, Calxeda, Cisco, Citrix, Google, Intel, Oracle, Samsung, and Verizon.^[17] The Xen project itself is self-governing.^[18]

Since version 3.0 of the Linux kernel, Xen support for dom0 and domU exists in the mainline kernel.^[19]