An old paper presentation held at Kolhapur Institute of Technology, India.

1. 1 --- Cooperative Linux
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COOPERATIVE LINUX
Mr. Ankit Singh
Final year, Computer Engineering,
AISSMS’s College of Engineering, Pune.
Email : ankit***@***.com
Contact no. : - +91-9******5
Mr. Sushant Bhadkamkar
Final year, Computer Engineering,
AISSMS’s College of Engineering, Pune.
Email : ***sushant@***.com
Contact no. : - +91-9*****4

2. 2 --- Cooperative Linux
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Abstract
In this paper we will describe Cooperative
Linux (short-named coLinux), an effective
way to run Linux under other operating
system such as Windows, Linux etc. The
paper briefly presents technical details as
to how cooperation between two operating
systems is achieved. We will compare
coLinux with other virtual machines
available and discuss the advantages and
the downsides of using coLinux. The paper
also discusses a few applications and
projects currently being carried out with
the help of coLinux.
Keywords
Virtualization, CVM (Cooperative Virtual
Machine), Host OS, Kernel.
1. Introduction
Operating system virtualization is popular
technology these days. People run
different operating systems on top of their
existing ones not only for experimental
purposes but also for production use.
Many different virtualization systems
exist. In 2004, a novel concept of co-
operative virtualization was introduced by
Mr. Dan Aloni which made it possible for
two operating systems to work in co-
operation, both having complete control of
the host machine.
1.1 Virtualization
Virtualization is a framework or
methodology of dividing the resources of a
computer into multiple execution
environments, by applying one or more
concepts or technologies such as hardware
and software partitioning, time-sharing,
partial or complete machine simulation,
emulation, quality of service, and many
others.
Figure 1.1: An Example of virtualization.
1.2 Cooperative Linux
Cooperative Linux, the first working free
and open source method for optimally
running Linux on Microsoft Windows
natively. More generally, Cooperative
Linux is a port of the Linux kernel that
allows it to run cooperatively alongside
another operating system on a single
machine. The term ‘Cooperative’ is used
to describe two Entities working in
parallel, e.g. co-routines. In that sense the
simplest description of Cooperative Linux
is turning two operating system kernels
into two big co-routines. In that mode,
each kernel has its own complete CPU
context and address space, and each kernel

3. 3 --- Cooperative Linux
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decides when to give control back to its
partner. However, only one of the two
kernels has control on the physical
hardware, where the other is provided only
with virtual hardware abstraction.
2. Implementation
2.1 Design of coLinux
coLinux is a port of the Linux kernel that
is modified to run cooperatively with
another operating system. Dan Aloni
describes coLinux as a cooperative and
paravirtualized Linux virtual machine.
Cooperative means that it gives control
back to the host OS at will. Paravirtualized
means that the coLinux kernel has no
notion of the real hardware except that of
the CPU and memory. The host operating
system (Windows or Linux) maintains
control of the physical resources of the
operating system, while the guest
operating system (coLinux) is provided
with a virtual abstraction of the hardware.
The host operating system must provide
the means to execute a driver in the
privileged ring (ring 0) and export the
means to allocate memory.
This is unlike VMware which intercepts
I/O to hardware devices and emulates it.
coLinux feels like a separate Linux box --
the guest kernel's internals are separated
from the host kernel's internals.
Figure 2.1. coLinux internals.
Notice that coLinux consists of two parts:
 The coLinux kernel driver that
operates in the host kernel space.
 Several user space daemons.
The coLinux kernel driver's main jobs are
to: Load the Linux guest kernel image on
start. You can imagine this functionality
being similar to a bootloader (like LILO or
GRUB).
 Execute ioctl() requests as
instructed by the coLinux-daemon
process. This ioctl() call is
responsible for doing context
switches between guest OS and
host OS.

4. 4 --- Cooperative Linux
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 Act as a forwarder of interrupts and
requests from several virtual
drivers: cobd (block device), conet
(network), and cocon (console).
In the user space, the most important part
is the coLinux-daemon-process. Besides
being responsible for triggering context
switches, it works as the "manager" of
several other daemons such as coLinux-
console-nt and coLinux-net-daemon. For
example, via coLinux-console, users can
see the current display of the active
console of the Linux guest. When the user
types or issues a command inside the shell
of this coLinux-console, the coLinux-
daemon will "wrap" it and forward it to
coLinux-driver.
Figure 2.2: Virtualization in coLinux
This concept is called ‘cooperative
virtualization’.
2.2 Device Driver
The device driver port of Cooperative
Linux is used for accessing kernel mode
and using the kernel primitives that are
exported by the host OS kernel. Most of
the driver is OS independent code that
interfaces with the OS dependent
primitives that include page allocations,
debug printing, and interfacing with user
space.
When a Cooperative Linux VM is created,
the driver loads a kernel image from a
vmlinux file that was compiled from the
patched kernel with
CONFIG_COOPERATIVE. The vmlinux file
doesn’t need any cross platform tools in
order to be generated, and the same
vmlinux file can be used to run a
Cooperative Linux VM on several OSes of
the same architecture.
3. Comparison with other VMs
In this section we will compare coLinux
with other Linux virtual machines.
 coLinux runs without any
hypervisor[1]
, it is quite faster, and
if you install it on standard
partition you can get some speed in
reading/writing to the disk.
 In contrast to coLinux, traditional
VMs are unprivileged and under
the complete control of the host
machine.
 We can execute Linux applications
on coLinux without rebuilding.
[1] Virtual machine monitor that
Arbitrates access to the physical hardware

5. 5 --- Cooperative Linux
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 coLinux is faster than other VMs
as coLinux allows the Linux kernel
to run natively (as a Windows
kernel driver).
4. Advantages
 Effective way for casual windows
users to install and explore
Linux.
Windows users who are reluctant to
use Linux and other open source
software because of the complicated
technicalities, can be encouraged to do
so because of the relative easiness of
using coLinux. Installing coLinux is
relatively easier if normal installation
instructions are followed.
 coLinux is portable
You can have a coLinux distribution
with a custom set of applications on a
given Windows host (within the root
file system). You can move the root
file system to another host, and then
restart it. This allows for a mobile
development platform where the
compressed root file system fits on a
standard Universal Serial Bus (USB)
memory stick.
 Using two OSes without
rebooting.
The people, who already use Linux,
will use coLinux as a convenience, in
order to be able to do said Linux things
in Windows without having to reboot.
 Linux kernel research
coLinux kernel can be used to study
the Linux kernel, the two being quite
similar
5. Disadvantages
 The primary disadvantage of
coLinux is that it has the ability
to crash the entire machine (all
cooperating operating systems).
The guest operating system runs in a
privileged mode in the host kernel. It
also has some dependencies on
external software for normal operation
(windows and networking support).
 coLinux cannot be run in NT 4.0.
This is because some system calls are
missing in NT 4 (even with SP6) and
present only in 2000, XP and 2003.
 Limited Graphics Support.
Graphics performance is also
dependent on the speed of the
connection between the coLinux OS
and the Hosting OS.
 Another disadvantage is security.
Acquiring root user access on a
Cooperative Linux machine can
potentially lead to root access on the
host machine. An attacker can load
specially crafted kernel module or find
some other way to inject code into the

6. 6 --- Cooperative Linux
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running coLinux kernel, causing the
entire system to crash.
6. Projects using coLinux
 IBM is attempting to combine
coLinux and openMosix to offer a
high-performance cluster
middleware in a heterogeneous
environment. In this heterogeneous
environment, Linux™ provides the
stability and performance, and
Windows® users continue to use
_their apps and never notice the
difference.
 A coLinux Cross Tools CD-ROM
has been available from SSV
Embedded Systems since June
2005 and makes it possible use
coLinux for software development
of DIL/NetPC-based embedded
Linux applications.
7. Future of coLinux
coLinux is still a project in its beta and has
a small developer community. The
community will have to decide whether it
wants to promote coLinux as tool for users
to try out Linux or as a convenience for
developers. If the former is the case then
coLinux will have to be made more user-
friendly. A lot more documentation will
have to provided for troubleshooting.
Developers on the other hand will require
for it to have latest distribution support and
support for architectures other than x86.
India on the other hand can benefit a great
deal from coLinux as more and more
enterprises attempt to migrate towards
Linux and open source software. coLinux
can be used as a training tool in academic
institutes and students can get involved in
the coLinux project development.
8. Conclusion
While there are many virtualization
schemes out there, coLinux is novel in its
approach and the capabilities that it
provides. coLinux is not perfect but the
exposure that Linux gets by the average
computer user can increase tremendously.
Moreover it is an open source tool. The
implications that Cooperative Linux has on
what the media defines as ‘Linux on the
Desktop’—are massive, as the world’s
most dominant albeit proprietary desktop
OS supports running Linux distributions
for free, as another software, with the aim
of possibility that the Linux newbie would
switch to the standalone Linux.
So, if you want an easy way of running
simple, not very graphics demanding
applications from Linux on Windows, you
are a developer who develops for both
systems, or if you are just a Linux
enthusiast who wants to have his beloved

7. 7 --- Cooperative Linux
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applications at hand while using Windows,
and you don’t mind occasional problems
with graphics – coLinux is the solution for
you.
References
1. www.kernel.org
2. http://colinux.wikia.com/wiki/Main
_Page
3. http://www.ibm.com/developerwor
ks/opensource/
4. coLinux 2004 publication – Dan
Aloni
5. Rute User’s Text and Exposition –
Paul Sheer
6. linux.colinux.devel
.