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author xenbk@gandalf.hpl.hp.com
date Tue Jun 22 08:54:26 2004 +0000 (2004-06-22)
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10 University of Cambridge Computer Laboratory
11 15 Jun 2004
13 http://www.cl.cam.ac.uk/netos/xen
15 About the Xen Virtual Machine Monitor
16 =====================================
18 "Xen" is a Virtual Machine Monitor (VMM) originally developed by the
19 Systems Research Group of the University of Cambridge Computer
20 Laboratory, as part of the UK-EPSRC funded XenoServers project.
22 The XenoServers project aims to provide a "public infrastructure for
23 global distributed computing", and Xen plays a key part in that,
24 allowing us to efficiently partition a single machine to enable
25 multiple independent clients to run their operating systems and
26 applications in an environment providing protection, resource
27 isolation and accounting. The project web page contains further
28 information along with pointers to papers and technical reports:
29 http://www.cl.cam.ac.uk/xeno
31 Xen has since grown into a project in its own right, enabling us to
32 investigate interesting research issues regarding the best techniques
33 for virtualizing resources such as the CPU, memory, disk and network.
34 The project has been bolstered by support from Intel Research
35 Cambridge, and HP Labs, who are now working closely with us. We're
36 also in receipt of support from Microsoft Research Cambridge to port
37 Windows XP to run on Xen.
39 Xen enables multiple operating system images to execute concurrently
40 on the same hardware with very low performance overhead --- much lower
41 than commercial offerings for the same x86 platform.
43 This is achieved by requiring OSs to be specifically ported to run on
44 Xen, rather than allowing unmodified OS images to be used. Crucially,
45 only the OS needs to be changed -- all of the user-level application
46 binaries, libraries etc can run unmodified. Hence the modified OS
47 kernel can typically just be dropped into any existing OS distribution
48 or installation.
50 Xen currently runs on the x86 architecture, but could in principle be
51 ported to others. In fact, it would have been rather easier to write
52 Xen for pretty much any other architecture as x86 is particularly
53 tricky to handle. A good description of Xen's design, implementation
54 and performance is contained in our October 2003 SOSP paper, available
55 at http://www.cl.cam.ac.uk/netos/papers/2003-xensosp.pdf
57 We have worked on porting 4 different operating systems to run
58 on Xen: Linux 2.4/2.6, Windows XP, NetBSD and FreeBSD.
60 The Linux 2.4 port (currently Linux 2.4.26) works very well -- we
61 regularly use it to host complex applications such as PostgreSQL,
62 Apache, BK servers etc. It runs every user-space applications we've
63 tried. We refer to our version of Linux ported to run on Xen as
64 "XenLinux", although really it's just standard Linux ported to a new
65 virtual CPU architecture that we call xen-x86.
67 NetBSD has been ported to Xen by Christian Limpach, and will hopefully
68 soon become part of the standard release. Work on a FreeBSD port has
69 been started by Kip Macy, and we hope to see this complete for the 1.3
70 release.
72 The Windows XP port is nearly finished. It's running user space
73 applications and is generally in pretty good shape thanks to some hard
74 work by the team over the summer. Of course, there are issues with
75 releasing this code to others. We should be able to release the
76 source and binaries to anyone that has signed the Microsoft academic
77 source license, which these days has very reasonable terms. We are in
78 discussions with Microsoft about the possibility of being able to make
79 binary releases to a larger user community. Obviously, there are
80 issues with product activation in this environment which need to be
81 thought through.
83 So, for the moment, you only get to run Linux 2.4/2.6 and NetBSD on
84 Xen, but we hope this will change before too long. Even running
85 multiple copies of the same OS can be very useful, as it provides a
86 means of containing faults to one OS image, and also for providing
87 performance isolation between the various OS, enabling you to either
88 restrict, or reserve resources for, particular VM instances.
90 It's also useful for development -- each version of Linux can have
91 different patches applied, enabling different kernels to be tried
92 out. For example, the "vservers" patch used by PlanetLab applies
93 cleanly to our ported version of Linux.
95 We've successfully booted over 128 copies of Linux on the same machine
96 (a dual CPU hyperthreaded Xeon box) but we imagine that it would be
97 more normal to use some smaller number, perhaps 10-20.
100 Hardware support
101 ================
103 Xen is intended to be run on server-class machines, and the current
104 list of supported hardware very much reflects this, avoiding the need
105 for us to write drivers for "legacy" hardware. It is likely that some
106 desktop chipsets will fail to work properly with the default Xen
107 configuration: specifying 'noacpi' or 'ignorebiostables' when booting
108 Xen may help in these cases.
110 Xen requires a "P6" or newer processor (e.g. Pentium Pro, Celeron,
111 Pentium II, Pentium III, Pentium IV, Xeon, AMD Athlon, AMD Duron).
112 Multiprocessor machines are supported, and we also have basic support
113 for HyperThreading (SMT), although this remains a topic for ongoing
114 research. We're also working on an AMD x86_64 port (though Xen should
115 run on Opterons in 32-bit mode just fine).
117 Xen can currently use up to 4GB of memory. It's possible for x86
118 machines to address more than that (64GB), but it requires using a
119 different page table format (3-level rather than 2-level) that we
120 currently don't support. Adding 3-level PAE support wouldn't be
121 difficult, but we'd also need to add support to all the guest
122 OSs. Volunteers welcome!
124 In contrast to previous Xen versions, in Xen 2.0 device drivers run
125 within a privileged guest OS rather than within Xen itself. This means
126 that we should be compatible with the full set of device hardware
127 supported by Linux. The default XenLinux build contains support for
128 relatively modern server-class network and disk hardware, but you can
129 add suppport for other hardware by configuring your XenLinux kernel in
130 the normal way (e.g. "make xconfig").
133 Building Xen and XenLinux
134 =========================
136 The public master BK repository for the 2.0 release lives at:
137 bk://xen.bkbits.net/xeno-2.0.bk
139 To fetch a local copy, install the BitKeeper tools, then run:
140 'bk clone bk://xen.bkbits.net/xeno-2.0.bk'
142 You can do a complete build of Xen, the control tools, and the
143 XenLinux kernel images with "make world". This can take 10 minutes
144 even on a fast machine. If you're on an SMP machine you may wish to
145 give the '-j4' argument to make to get a parallel build. You should
146 end up with all the binaries and images being placed in the ./install
147 directory tree. You can then install everything to the standard
148 system directories (e.g. /boot, /usr/bin, /usr/lib/python/ etc) by
149 taping "make install".
152 Inspect the Makefule if you want to see what goes on during a
153 build. Building Xen and the tools is straightforward, but XenLinux is
154 more complicated. The makefile needs a 'pristine' linux kernel tree
155 which it will then add the Xen architecture files to. You can tell the
156 makefile the location of the appropriate linux compressed tar file by
157 setting the LINUX_SRC environment variable
158 (e.g. "LINUX_SRC=/tmp/linux-2.4.26.tar.gz make world") or by placing
159 the tar file somewhere in the search path of LINUX_SRC_PATH which
160 defaults to ".:..". If the makefile can't find a suitable kernel tar
161 file it attempts to download it from kernel.org, but this won't work
162 if you're behind a firewall.
164 After untaring the pristine kernel tree, the makefile uses the
165 'mkbuildtree' script to add the Xen patches the kernel. "make world"
166 then build two different XenLinux images, one with a "-xen0" extension
167 which contains hardware device drivers and is intended to be used in
168 the first virtual machine ("domain 0"), and one with a "-xenU"
169 extension that just contains virtual-device drivers. The latter can be
170 used for all non hardware privileged domains, and is substantially
171 smaller than the other kernel with its selection of hardware drivers.
173 If you don't want to use bitkeeper to download the source, you can
174 download prebuilt binaries and src tar balls from the project
175 downloads page: http://www.cl.cam.ac.uk/netos/xen/downloads/
177 Using the domain control tools
178 ==============================
180 README.CD contains some example invocations.
182 See example Python scripts in tools/examples/ and the associated README.
184 Further documentation is in docs/ (e.g., docs/Xen-HOWTO), and also in