Running Xen.pdf - Mailing List

processor, unlike IBM mainframes, was not designed to support complete virtualization. Instead, the Disco
project developers used a technique later called paravirtualization to introduce targeted modifications to enable
virtualization. The Disco project developers modified and recompiled IRIX to allow it to run on the modified
virtual architecture.
The Stanford team turned its attention to modifying another commodity platform not designed for virtualization,
x86. This led directly to the founding of VMware and the company's introduction of the first commercial
virtualization product for x86. In this case, they enabled running unmodified operating system binaries, such as
Microsoft Windows, by performing on-the-fly binary translation for instructions not allowed in their modified x86
architecture. For example, the POPF instruction (which pops from the stack and stores the value into a flags
register) must be replaced because when it runs in unprivileged mode it dies silently without making the
requested changes to the interrupt-disable flags.
Virtualization Extensions for x86
Since 2005, processor manufacturers such as Intel and AMD have added additional hardware support to their
product lines. Intel Virtualization Technology (VT) was developed under the codename Vanderpool and AMD
Virtualization (AMD-V) under the codename Pacifica. This hardware furthers the goal of commodity virtualization
by adding explicit functionality to enable higher performance hypervisors for full virtualization. With these recent
hardware revisions, full virtualization is both easier to implement and has the potential for higher performance.
(Recall that Xen uses these extensions to support full virtualization.)
Xen Origins and Time Line
The Xen hypervisor originated at the University of Cambridge Computer Laboratory as part of the ongoing
XenoServer project in 2001. The XenoServer project aims to construct a public infrastructure for wide-area
distributed computing. The goal of the project is to create a system in which XenoServer execution platforms
will be scattered across the globe for use by any member of the public. When the XenoServer infrastructure is
completed, its users will submit code for execution and be billed later for any resources consumed during the
course of execution. To ensure each physical node is utilized to the fullest extent possible, a high-performance
hypervisor for hosting multiple commodity operating systems on a single x86-based server was needed. In that
capacity, Xen was created to form the core of each XenoServer node. Xen enables accounting, auditing, and
most importantly, the resource management required for the XenoServer infrastructure. For more information
on the XenoServer project see http://www.xenoservers.net/.
Xen was first publicly unveiled in an academic paper accepted into the 2003 proceedings of the Association for
Computing Machinery (ACM) Symposium on Operating Systems Principles (SOSP). The claims of fast
virtualization supported on commodity x86 machines garnered wide interest from the academic community.
Those claims were independently verified in an academic setting, thus strengthening the claims of performance.
Soon after, a growing number of parties became interested in this new take on virtualization. In the years since
the initial Xen publication, several major updates to the project have occurred, providing enhanced functionality,
reliability, and performance.
It is noteworthy that during the development of Xen 1.x, Microsoft Research, in collaboration with the University
of Cambridge Operating System group, developed a port of Windows XP to Xen. The port was made possible in
part by Microsoft's Academic Licensing Program. Unfortunately, due to the terms of that license program, the
port was never published, although it is mentioned in the original Xen SOSP paper.
A separate company, XenSource, was founded in 2004 to promote the ubiquitous adoption of an open source
Xen hypervisor in the enterprise. XenSource focused its business on supporting the development of the wholly
open sourced Xen core, while simultaneously selling enterprise packages and management software to its
customers. While XenSource leads and coordinates the development efforts, contributions have been made by a
wide array of companies and organizations including IBM, Sun, HP, Red Hat, Intel, AMD, SGI, Novell, the NSA,
the US Navy, Samsung, Fujitsu, Qlogic, and many others, including academic researchers from many
universities. Together, they produce a standard that all players in the Xen ecosystem can rely on, reducing risk
and speeding development for all participants.