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What is Xen?

Xen is a Virtual Machine Monitor (VMM) originally developed by the
Systems Research Group of the University of Cambridge Computer
Laboratory, as part of the UK-EPSRC funded XenoServers project.  Xen
is freely-distributable Open Source software, released under the GNU
GPL. Since its initial public release, Xen has grown a large
development community, spearheaded by (

The 4.5 release offers a number of improvements, including:
improvements for large scale machines during bootup and for PCI
passthrough; multiple IO-REQ servers (many QEMUs for a guest); soft
affinity for vCPUs (aka NUMA affinity); and API expansion for guest
introspection.  We also have number of updates for CPU specific
changes, such as: Broadwell Supervisor Mode Access Prevention; Haswell
Server Cache QoS Monitoring aka Intel Resource Director Technology;
further extensions to vAPIC (SandyBridge feature); fixes in AMD
microcode loading; Data Breaking Extensions; and further MSR masking
support on AMD.

On the experimental side we have added a new Real-Time Deferrable
Server Based CPU Scheduler (rtds), and PVH initial domain (dom0)
support for Intel CPUs.

Additionally, 4.5 has a huge update to the ARM code, including support
for: up to 1TB in guests; up to 8 CPUs; Power State Coordination
Interface (0.2) to power up and down CPUs; UEFI booting; IOMMU support
(SMMUv1); Super Page (2MB) support; passthrough of MMIO regions to
guests; and lower interrupt latency.

The toolstack has expanded to include support for: VM Generation ID (a
Windows 2012 Server requirement); Remus initial support (for high
availability) in libxl (since xend has been removed); libxenlight JSON
support, HVM guest direct kernel boot, and persistent configuration
support; systemd support; performance optimizations in oxenstored;
and support in QEMU for expanding the PCI hole.

Lastly, we have removed the Python toolstack (xend).

And as always, there are a number of performance, stability, and security
improvements under-the hood.

This file contains some quick-start instructions to install Xen on
your system. For more information see http:/ and

Quick-Start Guide

First, there are a number of prerequisites for building a Xen source
release. Make sure you have all the following installed, either by
visiting the project webpage or installing a pre-built package
provided by your OS distributor:
    * GCC v4.1 or later
    * GNU Make
    * GNU Binutils
    * Development install of zlib (e.g., zlib-dev)
    * Development install of Python v2.3 or later (e.g., python-dev)
    * Development install of curses (e.g., libncurses-dev)
    * Development install of openssl (e.g., openssl-dev)
    * Development install of x11 (e.g. xorg-x11-dev)
    * Development install of uuid (e.g. uuid-dev)
    * Development install of yajl (e.g. libyajl-dev)
    * Development install of libaio (e.g. libaio-dev) version 0.3.107 or
    * Development install of GLib v2.0 (e.g. libglib2.0-dev)
    * Development install of Pixman (e.g. libpixman-1-dev)
    * pkg-config
    * bridge-utils package (/sbin/brctl)
    * iproute package (/sbin/ip)
    * GNU bison and GNU flex
    * GNU gettext
    * 16-bit x86 assembler, loader and compiler (dev86 rpm or bin86 & bcc debs)
    * ACPI ASL compiler (iasl)

In addition to the above there are a number of optional build
prerequisites. Omitting these will cause the related features to be
disabled at compile time:
    * Development install of Ocaml (e.g. ocaml-nox and
      ocaml-findlib). Required to build ocaml components which
      includes the alternative ocaml xenstored.
    * cmake (if building vtpm stub domains)
    * markdown
    * figlet (for generating the traditional Xen start of day banner)
    * systemd daemon development files
    * Development install of libnl3 (e.g., libnl-3-200,
      libnl-3-dev, etc).  Required if network buffering is desired
      when using Remus with libxl.  See docs/README.remus for detailed

Second, you need to acquire a suitable kernel for use in domain 0. If
possible you should use a kernel provided by your OS distributor. If
no suitable kernel is available from your OS distributor then refer to for suggestions for
suitable kernels to use.
If you are looking to compile a Dom0 kernel from source, please refer to

[NB. Unless noted otherwise, all the following steps should be
performed with root privileges.]

1. Download and untar the source tarball file. This will be a
   file named xen-unstable-src.tgz, or xen-$version-src.tgz.
   You can also pull the current version from the git or mercurial
   repositories at

    # tar xzf xen-unstable-src.tgz

   Assuming you are using the unstable tree, this will
   untar into xen-unstable. The rest of the instructions
   use the unstable tree as an example, substitute the
   version for unstable.

2. cd to xen-unstable (or whatever you sensibly rename it to).

3. For the very first build, or if you want to destroy build trees,
   perform the following steps:

    # ./configure
    # make world
    # make install

   See the documentation in the INSTALL file for more info.

   This will create and install onto the local machine. It will build
   the xen binary (xen.gz), the tools and the documentation.

   You can override the destination for make install by setting DESTDIR
   to some value.

4. To rebuild an existing tree without modifying the config:
    # make dist

   This will build and install xen, tools, and docs into the local dist/

   You can override the destination for make install by setting DISTDIR
   to some value.

   make install and make dist differ in that make install does the
   right things for your local machine (installing the appropriate
   version of udev scripts, for example), but make dist includes all
   versions of those scripts, so that you can copy the dist directory
   to another machine and install from that distribution.

xenstore: xenstored and oxenstored

Xen uses a configuration database called xenstore [0] to maintain configuration
and status information shared between domains. A daemon is implemented as part
of xenstore to act as an interface for access to the database for dom0 and
guests. Two xenstored daemons are supported, one written in C which we refer
to as the xenstored (sometimes referred to as cxenstored), and another written
in Ocaml called oxenstored. Details for xenstore and the different
implementations can be found on the wiki's xenstore reference guide [1] and
the xenstored [2] page. You can choose which xenstore you want to enable as
default on a system through configure:

	./configure --with-xenstored=xenstored
	./configure --with-xenstored=oxenstored

By default oxenstored will be used if the ocaml development tools are found.
If you enable oxenstored the xenstored will still be built and installed,
the xenstored used can be changed through the configuration file:


You can change the preferred xenstored you want to use in the configuration
but since we cannot stop the daemon a reboot will be required to make the
change take effect.


Python Runtime Libraries

Various tools, such as pygrub, have the following runtime dependencies:

    * Python 2.3 or later.
          Debian: python

Intel(R) Trusted Execution Technology Support

Intel's technology for safer computing, Intel(R) Trusted Execution Technology
(Intel(R) TXT), defines platform-level enhancements that provide the building
blocks for creating trusted platforms.  For more information, see

Intel(R) TXT support is provided by the Trusted Boot (tboot) module in
conjunction with minimal logic in the Xen hypervisor.

Tboot is an open source, pre- kernel/VMM module that uses Intel(R) TXT to
perform a measured and verified launch of an OS kernel/VMM.

The Trusted Boot module is available from  This project hosts the code in a
mercurial repo at and contains
tarballs of the source.  Instructions in the tboot README describe how
to modify grub.conf to use tboot to launch Xen.

There are optional targets as part of Xen's top-level makefile that will
download and build tboot: install-tboot, build-tboot, dist-tboot, clean-tboot.
These will download the latest tar file from the SourceForge site using wget,
then build/install/dist according to Xen's settings.
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