Linux KVM as a Learning Tool

Low-level system programming is a difficult task, but with Linux KVM, it's a whole lot easier.

Build both the launcher and kernel2, and run them as usual. The output should be similar to this:

$ ./launcher kernel2

Now the top command should show 0% CPU usage for the launcher process, because its virtual CPU is halted.

As a last example, an improved kernel is shown in Listing 12, using the OUTSB string output instruction and the REP prefix to repeat it the number of times specified by CX. Interestingly, this code generates only one I/O exit to output the entire string. Compare this against the previous kernel2, which generates one I/O exit for each outb execution, with the associated overhead due to context switches. You can use the kvm_stat Python script from the KVM sources to see this and other behaviours of the virtual machines.

The CS prefix before the LEA and OUTSB instructions are needed to fetch data (greeting string) from the code segment.

What's Next?

At this point, you have the basis to experiment with all kinds of real-mode code. You can extend the examples to set an IDT and handle interrupts or add more I/O devices. A good starting point is interrupts to learn the constraints of interrupt context, and another one is to investigate the rest of LibKVM's methods.

However, real mode is not enough to learn all the things that current kernels do on the x86 platform. For this reason, in a follow-up article, we will extend our launcher a little in order to handle kernels running in 32-bit protected mode. This change will give us the ability to write kernels in the C language, allowing for rapid development of bigger kernels. It also will open the door for experimenting with segmentation, paging, privilege levels (two or more rings) and more.

Remember, low-level system programming is a challenging task, but with Linux KVM, it can be easy. So, go ahead and code, have fun and you will learn a lot about how computer systems work in the process!

Duilio Javier Protti ( is a software engineer with Intel Corp., in Cordoba, Argentina. He currently is working on a team specializing in virtualization technology. Before joining Intel, he wrote LibCMT (a library for composable memory transactions), was the maintainer of the Infinity XMMS plugin and contributed to various open-source projects, such as Nmap, Libvisual and others.



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Exit instead of hlt

Curious's picture

Thanks for this excellent article. I've tested out the sample but the program never returns after the kvm_run() call. I guess this is because the vcpu is halted in the last instruction of the test program. But how do I exit the KVM altogether and resume execution from kvm_run() onwards? Is there any documentation for libkvm somewhere I can consult?

This appears to be the libkvm in question, no?

Ben Scherrey's picture

See the userspace git tree. Is this the lib we need to be building to follow this article? I've been looking to do something like this with KVM for a while to create my own forth-like environment without having to screw around with low level hardware (or at least put it off til something interesting already works). Look forward to the follow up article. When is it scheduled?

-- Ben Scherrey

LibKVM on Ubuntu? Nonesuch...

basicman's picture

My attempt to follow this tutorial died on page 2, when it announced that all of the examples would be using the LibKVM library. Several hours of searching on Google and I've found nothing; no way to install or use the LibKVM library unless I'm on BSD.

If anyone has a workaround for this, I'd love to hear it. I was really looking forward to following the tutorial.

Different libkvm

Mitch Frazier's picture

That's a different library. You need to install the qemu-kvm-devel package. Note you will probably have to get it directly from sourceforge since it does not appear to be in the Ubuntu repos. I don't use Ubuntu much so maybe I'm overlooking something, however, I know that openSUSE does not have a package for it either (at least in the standard places at 11.0). Make sure that you get the version that corresponds to the version of kvm that you have installed. For example, here is a link to the -devel package for release 88.

If you have to install it from sourceforge some fiddling around will probably be required. First, you'll have to build it, then install it, and then potentially modify your include/library paths to find the needed items.

Mitch Frazier is an Associate Editor for Linux Journal.

I used REHL5.5 installed devel package for release 88

jniu's picture

I used REHL5.5 installed devel package for release 88, but still cannot find libkvm.h and libkvm.a, the installation was successful, can you give me some suggestion? thanks.