doc: Document how to repair Guix System from a chroot.

* doc/guix.texi (System Troubleshooting Tips): New chapter.

doc/guix.texi

@@ -169,6 +169,7 @@ Weblate} (@pxref{Translating Guix}).
 * Introduction::                What is Guix about?
 * Installation::                Installing Guix.
 * System Installation::         Installing the whole operating system.
+* System Troubleshooting Tips:: When things don't go as planned.
 * Getting Started::             Your first steps.
 * Package Management::          Package installation, upgrade, etc.
 * Channels::                    Customizing the package collection.
@@ -227,6 +228,10 @@ System Installation
 * Installing Guix in a VM::     Guix System playground.
 * Building the Installation Image::  How this comes to be.
 
+System Troubleshooting Tips
+
+* Chrooting into an existing system::  Fixing things from a chroot
+
 Manual Installation
 
 * Keyboard Layout and Networking and Partitioning:: Initial setup.
@@ -2368,6 +2373,7 @@ See the files under @file{/run/current-system/profile/share/keymaps} for
 a list of available keyboard layouts.  Run @command{man loadkeys} for
 more information.
 
+@anchor{manual-installation-networking}
 @subsubsection Networking
 
 Run the following command to see what your network interfaces are called:
@@ -2821,6 +2827,119 @@ guix system image --system=armhf-linux -e '((@@ (gnu system install) os-with-u-b
 @code{A20-OLinuXino-Lime2} is the name of the board.  If you specify an invalid
 board, a list of possible boards will be printed.
 
+@c *********************************************************************
+@cindex troubleshooting, guix system
+@cindex guix system troubleshooting
+@node System Troubleshooting Tips
+@chapter System Troubleshooting Tips
+
+Guix System allows rebooting into a previous generation should the last
+one be malfunctioning, which makes it quite robust against being broken
+irreversibly.  This feature depends on GRUB being correctly functioning
+though, which means that if for whatever reasons your GRUB installation
+becomes corrupted during a system reconfiguration, you may not be able
+to easily boot into a previous generation.  A technique that can be used
+in this case is to @i{chroot} into your broken system and reconfigure it
+from there.  Such technique is explained below.
+
+@cindex chroot, guix system
+@cindex chrooting, guix system
+@cindex repairing GRUB, via chroot
+@node Chrooting into an existing system
+@section Chrooting into an existing system
+
+This section details how to @i{chroot} to an already installed Guix
+System with the aim of reconfiguring it, for example to fix a broken
+GRUB installation.  The process is similar to how it would be done on
+other GNU/Linux systems, but there are some Guix System particularities
+such as the daemon and profiles that make it worthy of explaining here.
+
+@enumerate
+@item
+Obtain a bootable image of Guix System.  It is recommended the latest
+development snapshot so the kernel and the tools used are at least as as
+new as those of your installed system; it can be retrieved from the
+@url{https://ci.guix.gnu.org/search/latest/ISO-9660?query=spec:images+status:success+system:x86_64-linux+image.iso,
+https://ci.guix.gnu.org} URL.  Follow the @pxref{USB Stick and DVD
+Installation} section for copying it to a bootable media.
+
+@item
+Boot the image, and proceed with the graphical text-based installer
+until your network is configured.  Alternatively, you could configure
+the network manually by following the
+@ref{manual-installation-networking} section.  If you get the error
+@samp{RTNETLINK answers: Operation not possible due to RF-kill}, try
+@samp{rfkill list} followed by @samp{rfkill unblock 0}, where @samp{0}
+is your device identifier (ID).
+
+@item
+Switch to a virtual console (tty) if you haven't already by pressing
+simultaneously the @kbd{Control + Alt + F4} keys.  Mount your file
+system at @file{/mnt}.  Assuming your root partition is
+@file{/dev/sda2}, you would do:
+
+@example sh
+mount /dev/sda2 /mnt
+@end example
+
+@item
+Mount special block devices and Linux-specific directories:
+
+@example sh
+mount --bind /proc /mnt/proc
+mount --bind /sys /mnt/sys
+mount --bind /dev /mnt/dev
+@end example
+
+If your system is EFI-based, you must also mount the ESP partition.
+Assuming it is @file{/dev/sda1}, you can do so with:
+
+@example sh
+mount /dev/sda1 /mnt/boot/efi
+@end example
+
+@item
+Enter your system via chroot:
+
+@example sh
+chroot /mnt /bin/sh
+@end example
+
+@item
+Source your @var{user} profile to setup the environment, where
+@var{user} is the user name used for the Guix System you are attempting
+to repair:
+
+@example sh
+source /home/@var{user}/.guix-profile/etc/profile
+@end example
+
+To ensure you are working with the Guix revision you normally would as
+your normal user, also source your current Guix profile:
+
+@example sh
+source /home/@var{user}/.config/guix/current/etc/profile
+@end example
+
+@item
+Start a minimal @command{guix-daemon} in the background:
+
+@example sh
+guix-daemon --build-users-group=guixbuild --disable-chroot &
+@end example
+
+@item
+Edit your Guix System configuration if needed, then reconfigure with:
+
+@example sh
+guix system reconfigure your-config.scm
+@end example
+
+@item
+Finally, you should be good to reboot the system to test your fix.
+
+@end enumerate
+
 @c *********************************************************************
 @node Getting Started
 @chapter Getting Started