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INTRODUCTION

Cooperative Linux utilizes the rather underused concept of a Cooperative Virtual Machine (CVM), in contrast to traditional VMs that are unprivileged and being under the complete control of the host machine. The term Cooperative is used to describe two entities working in parallel, e.g. coroutines [2]. In that sense the most plain description of Cooperative Linux is turning two operating system kernels into two big coroutines. In that mode, each kernel has its own complete CPU context and address space, and each kernel 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. From this point on in the paper I'll refer to these two kernels as the host operating system, and the guest Linux VM respectively. The host can be every OS kernel that exports basic primitives that provide the Cooperative Linux portable driver to run in CPL0 mode (ring 0) and allocate memory. The special CPL0 approach in Cooperative Linux makes it significantly different than traditional virtualization solutions such as VMware, plex86, Virtual PC, and other methods such as Xen. All of these approaches work by running the guest OS in a less privileged mode than of the host kernel. This approach allowed for the extensive simplification of Cooperative Linux's design and its short earlybeta development cycle which lasted only one month, starting from scratch by modifying the vanilla Linux 2.4.23-pre9 release until reaching to the point where KDE could run.

The only downsides to the CPL0 approach is stability and security. If it's unstable, it has the potential to crash the system. However, measures can be taken, such as cleanly shutting it down on the first internal Oops or panic. Another disadvantage is security. Acquiring root user access on a Cooperative Linux machine can potentially lead to root on the host machine if the attacker loads specially crafted kernel module or uses some very elaborated exploit in case which the Cooperative Linux kernel was compiled without module support.

One Most of the changes in the Cooperative Linux patch are on the i386 tree-the only supported architecture for Cooperative at the time of this writing. The other changes are mostly additions of virtual drivers: cobd (block device), conet (network), and cocon (console). Most of the changes in the i386 tree involve the initialization and setup code. It is a goal of the Cooperative Linux kernel design to remain as close as possible to the standalone i386 kernel, so all changes are localized and minimized as much as possible.

2. USES

Cooperative Linux in its current early state can already provide some of the uses that User Mode Linux[1] provides, such as virtual hosting, kernel development environment, research, and testing of new distributions or buggy software. It also enabled new uses:

Relatively effortless migration path from Windows.
In the process of switching to another OS, there is the choice between installing another computer, dualbooting, or using a virtualization software. The first option costs money, the second is tiresome in terms of operation, but the third can be the most quick and easy method-especially if it's free. This is where Cooperative Linux comes in. It is already used in workplaces to convert Windows users to Linux.

Adding Windows machines to Linux clusters.
The Cooperative Linux patch is minimal and can be easily combined with others such as the MOSIX or Open-MOSIX patches that add clustering capabilities to the kernel. This work in progress allows to addWindows machines to super-computer clusters, where one illustration could tell about a secretary workstation computer that runs Cooperative Linux as a screen saver-when the secretary goes home at the end of the day and leaves the computer unattended, the office's cluster gets more CPU cycles for free.

Running an otherwise-dual-booted Linux system from the other OS.
The Windows port of Cooperative Linux allows it to mount real disk partitions as block devices. Numerous people are using this in order to access, rescue, or just run their Linux system from their ext3 or reiserfs file systems.

Using Linux as a Windows firewall on the same machine.
As a likely competitor to other out-of-the-box Windows firewalls, iptables along with a stripped-down Cooperative Linux system can potentially serve as a network firewall.

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