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author iap10@labyrinth.cl.cam.ac.uk
date Fri Sep 19 18:26:35 2003 +0000 (2003-09-19)
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10 University of Cambridge Computer Laboratory
11 31 Aug 2003
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) developed by the Systems
19 Research Group of the University of Cambridge Computer Laboratory, as
20 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, who are now working closely with us. We're also in receipt
36 of support from Microsoft Research Cambridge to port Windows XP to
37 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 been working on porting 3 different operating systems to run
58 on Xen: Linux 2.4, Windows XP, and NetBSD.
60 The Linux 2.4 port (currently Linux 2.4.22) 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 "XenoLinux", although really it's just standard Linux ported to a new
65 virtual CPU architecture that we call xeno-x86 (abbreviated to just
66 "xeno").
68 Unfortunately, the NetBSD port has stalled due to lack of man
69 power. We believe most of the hard stuff has already been done, and
70 are hoping to get the ball rolling again soon. In hindsight, a FreeBSD
71 4 port might have been more useful to the community. Any volunteers? :-)
73 The Windows XP port is nearly finished. It's running user space
74 applications and is generally in pretty good shape thanks to some hard
75 work by the team over the summer. Of course, there are issues with
76 releasing this code to others. We should be able to release the
77 source and binaries to anyone that has signed the Microsoft academic
78 source license, which these days has very reasonable terms. We are in
79 discussions with Microsoft about the possibility of being able to make
80 binary releases to a larger user community. Obviously, there are
81 issues with product activation in this environment which need to be
82 thought through.
84 So, for the moment, you only get to run multiple copies of Linux on
85 Xen, but we hope this will change before too long. Even running
86 multiple copies of the same OS can be very useful, as it provides a
87 means of containing faults to one OS image, and also for providing
88 performance isolation between the various OS, enabling you to either
89 restrict, or reserve resources for, particular VM instances.
91 It's also useful for development -- each version of Linux can have
92 different patches applied, enabling different kernels to be tried
93 out. For example, the "vservers" patch used by PlanetLab applies
94 cleanly to our ported version of Linux.
96 We've successfully booted over 128 copies of Linux on the same machine
97 (a dual CPU hyperthreaded Xeon box) but we imagine that it would be
98 more normal to use some smaller number, perhaps 10-20.
102 Hardware support
103 ================
105 Xen is intended to be run on server-class machines, and the current
106 list of supported hardware very much reflects this, avoiding the need
107 for us to write drivers for "legacy" hardware.
109 Xen requires a "P6" or newer processor (e.g. Pentium Pro, Celeron,
110 Pentium II, Pentium III, Pentium IV, Xeon, AMD Athlon, AMD Duron).
111 Multiprocessor machines are supported, and we also have basic support
112 for HyperThreading (SMT), although this remains a topic for ongoing
113 research. We're also looking at an AMD x86_64 port (though it should
114 run on Opterons in 32-bit mode just fine).
116 Xen can currently use up to 4GB of memory. It's possible for x86
117 machines to address more than that (64GB), but it requires using a
118 different page table format (3-level rather than 2-level) that we
119 currently don't support. Adding 3-level PAE support wouldn't be
120 difficult, but we'd also need to add support to all the guest
121 OSs. Volunteers welcome!
123 We currently support a relatively modern set of network cards: Intel
124 e1000, Broadcom BCM 57xx (tg3), 3COM 3c905 (3c59x). Adding support for
125 other NICs that support hardware DMA scatter/gather from half-word
126 aligned addresses is relatively straightforward, by porting the
127 equivalent Linux driver. Drivers for a number of other older cards
128 have recently been added [pcnet32, e100, tulip], but are untested and
129 not recommended.
132 Building Xen and XenoLinux
133 ==========================
135 Take a look at the tools/misc/xen-clone script in the BK repository,
136 which will 'bk clone' the live master tree, and then set about
137 building everything. The build procedure for xenolinux is slightly
138 complicated as its done by running the 'mkbuildtree' script over
139 a pristine Linux tree to turn it into a xenolinux tree by adding the
140 'xeno' architecture.
142 The public master BK repository lives at: bk://xen.bkbits.net/xeno.bk
146 Ian Pratt
147 9 Sep 2003