Virtualization with coLinux

Cooperation is probably the last thing you think of when considering GNU/Linux and Microsoft Windows, but that's exactly what you get with the coLinux kernel. Not convinced? Skip ahead to "Advantages of coLinux."

coLinux is a port of the Linux operating system that executes as a single process in the Microsoft operating system. The operating systems cooperate with each other by giving each other the central processing unit (CPU), as shown in Figure 1.

Figure 1. Microsoft Windows and Linux cooperate with each other in coLinux

Through the use of kernel drivers, physical resources such as the networking interface are also shared. More on this later. First, what is meant by virtualization?

Virtualization

The spectrum of virtualization

This article focuses on a cooperative solution to platform virtualization, but the term applies to other techniques as well. For example:

• • In storage, you can virtualize many disks to look like a pool of storage that can be allocated based on need.

  • You can also virtualize networks, though this is usually called zoning.

  • OS virtualization, which is a simpler form of virtualization, virtualizes servers running on an operating system.

Virtualization is an over-used term. In the context of this article, I'm referring to the platform variation. Virtualizing a platform (or hardware) means that the hardware is abstracted from a physical platform into a collection of logical platforms onto which operating systems can be run. In the simplest sense, this means that you can run multiple operating systems (of the same or different types) on the same hardware platform.

The element of the system that provides the virtualization is commonly known as a virtual machine monitor or hypervisor. Each operating system uses its own virtual machine that cooperates with the hypervisor to arbitrate access to the physical hardware (see Figure 2).

Figure 2. The hypervisor arbitrates access to the physical hardware

Like operating systems, the methods available for virtualization are wide and varied. For more information about virtualization, see the Resources section.

coLinux

coLinux is a port of the standard Linux kernel. In other words, coLinux is the Linux kernel that's modified to run cooperatively with another operating system. 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 (see Figure 3).

Figure 3. coLinux executes as a process of the host operating system

The root file system for coLinux is a regular file within the host operating system. To Windows it's just a regular file, but to coLinux it's an ext3 file system that can be read and written to.

Other features needed by the Linux kernel, such as networking or video access, are proxied externally. Networking is made accessible to coLinux through a TUN/TAP driver (which is covered in "Networking," below). In short, this driver provides user-space access to the Ethernet device so that packets can be transmitted and received. Access to the display is also proxied. Recall that X Window System is a protocol by which video output can be sent from one host to another. Therefore, by using an X Window System server on the host operating system, video output can be redirected to the available X server.

Now on to the installation of coLinux and configuration for both networking and video display.

Installing coLinux

Installing coLinux is surprisingly simple. There are a few steps involved, depending upon what you intend to do, but they're straightforward and worked on my Windows XP box without a single problem. This section explores installing coLinux and enabling services such as networking.

The first step is to download a coLinux distribution. Go to http://www.colinux.org and select Downloads from the left sidebar. If your browser doesn't take you to SourceForge, select the link to go there directly. Near the middle of the page is the coLinux-stable package. Download the coLinux executable file (at the time of this writing, the latest is coLinux-0.6.4.exe). The 0.6.4 release of coLinux is the 2.6.11 Linux kernel. When it has finished downloading, double-click the file to install.

After the usual license acceptance, you'll be asked for the components that you'd like to install. Leave these as is (all should be selected), including downloading a root file system image. To make things easier later on, change the destination folder for the coLinux install to c:\colinux\ because colinux is the standard install subdirectory from the perspective of configuration files.

When the installation program asks for a root file system image, select the Debian distribution because it's the smallest and extracts to only 1GB. Select Install to perform the install and root file system download.

When the install completes, you're not quite done yet. The next step is to open a folder to the install subdirectory to decompress the root file system.

The coLinux README file includes lots of additional information about other install options. This is available in the install subdirectory (c:\colinux). There will be an oddly named file that ends in .bz2. Rename this file to root_fs (this is the default root file system file in the configuration).

At this point, installation of coLinux is basically done. You can then start the coLinux daemon to run with Windows XP as follows (invoking through a Command Prompt window):

After invoking the colinux-daemon, the boot window is displayed (see Figure 4). This provides the same boot information that you find in the traditional Linux boot. Notice that coLinux boots extremely fast.

Figure 4. The coLinux boot window

A console window is also produced (see Figure 5) that attaches to the colinux-daemon. From this window, you can log in to coLinux to interact with the shell. The default username and password is root/root.

Figure 5. The coLinux virtual console (shell)

From Figure 5, you can see that the coLinux console is made up of two sections: the traditional console and the virtual console that provides information about the monitor.

Networking

Networking support for coLinux is done from the Linux perspective when the install is complete. Recall that the TAP driver is loaded during the install. The TAP driver is a user-space tap onto the Ethernet device managed by the host operating system. The TAP driver allows the guest operating system to read or write raw Ethernet frames to a virtual Ethernet device (extended to user space). The virtual Ethernet device in user space then moves Ethernet frames to and from the real Ethernet device in the host operating system. Note that the TAP driver moves Ethernet frames, while the TUN driver is used for Internet Protocol (IP) frames.

For the TAP driver to work, the host operating system must share the available Ethernet device. To enable sharing within Windows XP, open the Network Connections panel from the Control Panel. Select the active local area connection, and then open the properties. Select the Advanced tab, and then select the check box for allowing other network users to connect through this computer's Internet connection (see Figure 6).

Figure 6. Local area connection properties for enabling network device sharing

After the network is shared, you can start coLinux and use the network as you would normally. This is shown in Figure 7 with the ping command.

Figure 7. Using the shared network device is transparent with coLinux through the TAP driver

With little effort and configuration, coLinux provides networking out of the box. For more information about networking with TUN/TAP, check out the Resources section.

X Window System

A console window is fine, but a graphical window manager would be ideal. With an open source X server, such as Xming, you can create xterms or use other graphical applications with coLinux. There are numerous documented options, such as Virtual Network Computing (VNC), but I'll show you how using Xming.

The first step is to download Xming from SourceForge. After installing, there's one file, called X0.hosts, that you need to update in the install subdirectory. It contains the remote hosts that are permitted access to the X server. Simply add the IP address of the machine that hosts the coLinux process. Start the Xming X server and coLinux, and then perform the following commands in coLinux:

Note that the IP address specified here is the IP address of the coLinux host. After you execute the xterm command, a new xterm window is presented, as shown in Figure 8.

Figure 8. The xterm created with Xming

Extending coLinux

Whichever root file system you download, it may not include everything that you'd like. But you can easily extend the root file system. For example, with Debian GNU/Linux, you can use the Advanced Packaging Tool (APT), a package management system, to install new packages or update existing ones. The first step is to update the APT metadata, which maintains management information about the installed packages (including newly available packages and where to get them):

Now you can update your root file system with other packages that you need. For example, if you want to add the wonderful Ruby language to your root file system, you invoke the following command:

In this way, you can use an existing root file system and tailor it to your specific needs. You can also rebuild coLinux from the sources if it lacks something that you need.

Advantages of coLinux

coLinux is a great way to use and experiment with Linux. Like Cygwin, it allows you to develop and execute Linux applications on the Windows operating system (through coLinux). You can also maintain the Linux operating system by installing, upgrading, or removing applications with apt-get.

Unlike Cygwin, you can execute Linux applications on coLinux without rebuilding. In this respect, coLinux is a real Linux operating system that runs (or cooperates) with the Windows operating system.

Another interesting advantage of coLinux is its portability. 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.

Finally, coLinux is fast because it's essentially running on the native hardware.

Problems with coLinux

The primary disadvantage of coLinux is that it has the ability to crash the entire machine (all cooperating operating systems) because 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). Outside of this, it's quite easy to install and configure. In the many hours that I've used it, I've never seen a crash.

Summary

While there are many virtualization schemes out there, coLinux is novel in its approach and the capabilities that it provides. coLinux by itself provides a virtualized Linux on top of Windows. With the addition of some other open source tools, you can support a full-fledged Linux development system complete with networking and a graphical user interface. coLinux isn't perfect, but it's a great way to use Linux on a standard Windows desktop computer.

Resources

Learn

• • The coLinux Web site gives the current status of the coLinux open source code for allowing a Linux distribution to run cooperatively on top of Windows.

  • The coLinux wiki provides documentation and other resources for getting started with coLinux.

  • Dan Aloni's Cooperative Linux (PDF) paper explores some of the lower-level technical details of the coLinux implementation.

  • "Virtual Linux" (developerWorks, December 2006) explores the history of virtualization and the various methods of virtualization available for Linux.

  • "An Introduction to Virtualization" by Amit Singh is a great introduction to virtualization and the varying implementations that are available. In addition to Linux, this article also covers other operating system virtualization (Windows, Solaris) and other means of virtualization (such as network stack virtualization).

  • TUN and TAP are virtual network kernel drivers.

  • Xen is an open source virtual machine monitor (hypervisor) developed by XenSource. Xen uses what is known as paravirtualization, which typically requires guest operating system modification, but Xen can take advantage of virtualization CPUs to support unmodified guest operating systems.

  • APT is a package management system from the Debian Linux distribution. It's a great utility to keep your system up to date.

  • In the developerWorks Linux zone, find more resources for Linux developers.

  • Stay current with developerWorks technical events and Webcasts.

Get products and technologies

• • Get the latest source and binary drops for coLinux at SourceForge.

  • Get the latest TUN/TAP driver at SourceForge.

  • KVM is a newcomer to virtualization for Linux. It's a kernel module solution that requires minimal changes to the Linux kernel and loads as a kernel module. KVM provides the virtualization of a guest operating system, but it requires QEMU for I/O. Read about the introduction of KVM into the 2.6.20 kernel.

  • Cygwin is another interesting Linux emulator that runs on Windows. coLinux can use the X server capabilities provided by Cygwin.

  • Xming is an open source X windows server for the Microsoft Windows environment.

  • Order the SEK for Linux, a two-DVD set containing the latest IBM trial software for Linux from DB2®, Lotus®, Rational®, Tivoli®, and WebSphere®.

  • With IBM trial software, available for download directly from developerWorks, build your next development project on Linux.

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About the author

M. Tim Jones is an embedded software architect and the author of GNU/Linux Application Programming, AI Application Programming, and BSD Sockets Programming from a Multilanguage Perspective. His engineering background ranges from the development of kernels for geosynchronous spacecraft to embedded systems architecture and networking protocols development. Tim is a Consultant Engineer for Emulex Corp. in Longmont, Colorado.