1. Introduction

Welcome to User Mode Linux. It's going to be fun.

1.1 How is User Mode Linux Different?

Normally, the Linux Kernel talks straight to your hardware (video card, keyboard, hard drives, etc), and any programs which run ask the kernel to operate the hardware, like so:

  +-----------+-----------+----+
  | Process 1 | Process 2 | ...|
  +-----------+-----------+----+
  |       Linux Kernel         |
  +----------------------------+
  |         Hardware           |
  +----------------------------+

The User Mode Linux Kernel is different; instead of talking to the hardware, it talks to a `real' Linux kernel (called the `host kernel' from now on), like any other program. Programs can then run inside User-Mode Linux as if they were running under a normal kernel, like so:

              +----------------+
              | Process 2 | ...|
  +-----------+----------------+
  | Process 1 | User-Mode Linux|
  +----------------------------+
  |       Linux Kernel         |
  +----------------------------+
  |         Hardware           |
  +----------------------------+

1.2 Why Would I Want User Mode Linux?

1. If User Mode Linux crashes, your host kernel is still fine.
2. You can run a usermode kernel as a non-root user.
3. You can debug the User Mode Linux like any normal process.
4. You can run gprof (profiling) and gcov (coverage testing).
5. You can play with your kernel without breaking things.
6. You can use it as a sandbox for testing new apps.
7. You can try new development kernels safely.
8. You can run different distributions simultaneously.
9. It's extremely fun.