docs: virt: user_mode_linux.rst: update compiling instructions

=====================

:Author:  User Mode Linux Core Team

:Last-updated: Mon Nov 18 14:16:16 EST 2002

:Last-updated: Sat Jan 25 16:07:55 CET 2020

This document describes the use and abuse of Jeff Dike's User Mode

Linux: a port of the Linux kernel as a normal Intel Linux process.

  Compiling the user mode kernel is just like compiling any other

  kernel.  Let's go through the steps, using 2.4.0-prerelease (current

  as of this writing) as an example:

  1. Download the latest UML patch from

     the download page <http://user-mode-linux.sourceforge.net/

     In this example, the file is uml-patch-2.4.0-prerelease.bz2.

  kernel.

  2. Download the matching kernel from your favourite kernel mirror,

  1. Download the latest kernel from your favourite kernel mirror,

     such as:

     ftp://ftp.ca.kernel.org/pub/kernel/v2.4/linux-2.4.0-prerelease.tar.bz2

     https://mirrors.edge.kernel.org/pub/linux/kernel/v5.x/linux-5.4.14.tar.xz

  3. Make a directory and unpack the kernel into it::

  2. Make a directory and unpack the kernel into it::

       host%

       mkdir ~/uml

       cd ~/uml

       host%

       tar -xzvf linux-2.4.0-prerelease.tar.bz2

  4. Apply the patch using::

       host%

       cd ~/uml/linux

       host%

       bzcat uml-patch-2.4.0-prerelease.bz2 | patch -p1

       tar xvf linux-5.4.14.tar.xz

  5. Run your favorite config; ``make xconfig ARCH=um`` is the most

  3. Run your favorite config; ``make xconfig ARCH=um`` is the most

     convenient.  ``make config ARCH=um`` and ``make menuconfig ARCH=um``

     will work as well.  The defaults will give you a useful kernel.  If

     you want to change something, go ahead, it probably won't hurt

  6. Finish with ``make linux ARCH=um``: the result is a file called

  4. Finish with ``make linux ARCH=um``: the result is a file called

     ``linux`` in the top directory of your source tree.

  Make sure that you don't build this kernel in /usr/src/linux.  On some

  distributions, /usr/include/asm is a link into this pool.  The user-

  mode build changes the other end of that link, and things that include

  <asm/anything.h> stop compiling.

  The sources are also available from cvs at the project's cvs page,

  which has directions on getting the sources. You can also browse the

  CVS pool from there.

  If you get the CVS sources, you will have to check them out into an

  empty directory. You will then have to copy each file into the

  corresponding directory in the appropriate kernel pool.

  If you don't have the latest kernel pool, you can get the

  corresponding user-mode sources with::

       host% cvs co -r v_2_3_x linux

  where 'x' is the version in your pool. Note that you will not get the

  bug fixes and enhancements that have gone into subsequent releases.

2.2.  Compiling and installing kernel modules

---------------------------------------------

3.1.  Running UML

-----------------

  It runs on 2.2.15 or later, and all 2.4 kernels.

  It runs on 2.2.15 or later, and all kernel versions since 2.4.

  Booting UML is straightforward.  Simply run 'linux': it will try to