Linux KVM as a Learning Tool

#ifndef __RUNTIME_H__
#define __RUNTIME_H__

// port to use for general purpose output
#define IO_PORT_PSEUDO_SERIAL  0xf1

#endif /* __RUNTIME_H_ */

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

$ ./launcher kernel2
Hello

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.

#include "runtime.h"

.code16
start:
    mov     $(IO_PORT_PSEUDO_SERIAL), %dx
    cs lea  greeting, %si
    mov     $14, %cx
    cs rep/outsb    // kvm_stat reports only
                    // *one* io_exit using this
    hlt

.align 16
greeting:
    .asciz  "Hello, World!\n"

. = 0xfff0
    ljmp    $0xf000, $start

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

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!