doc: cookbook: Add "Guix System Image API" section.

* doc/guix-cookbook.texi (Guix System Image API): New section.

Signed-off-by: Mathieu Othacehe <othacehe@gnu.org>
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Joshua Branson 2021-01-21 07:22:07 -05:00 committed by Mathieu Othacehe
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@ -1354,6 +1354,7 @@ reference.
@menu
* Customizing the Kernel:: Creating and using a custom Linux kernel on Guix System.
* Guix System Image API:: Customizing images to target specific platforms.
* Connecting to Wireguard VPN:: Connecting to a Wireguard VPN.
* Customizing a Window Manager:: Handle customization of a Window manager on Guix System.
* Running Guix on a Linode Server:: Running Guix on a Linode Server
@ -1602,6 +1603,217 @@ likely that you'll need to modify the initrd on a machine using a custom
kernel, since certain modules which are expected to be built may not be
available for inclusion into the initrd.
@node Guix System Image API
@section Guix System Image API
Historically, Guix System is centered around an @code{operating-system}
structure. This structure contains various fields ranging from the
bootloader and kernel declaration to the services to install.
Depending on the target machine, that can go from a standard
@code{x86_64} machine to a small ARM single board computer such as the
Pine64, the image constraints can vary a lot. The hardware
manufacturers will impose different image formats with various partition
sizes and offsets.
To create images suitable for all those machines, a new abstraction is
necessary: that's the goal of the @code{image} record. This record
contains all the required information to be transformed into a
standalone image, that can be directly booted on any target machine.
@lisp
(define-record-type* <image>
image make-image
image?
(name image-name ;symbol
(default #f))
(format image-format) ;symbol
(target image-target
(default #f))
(size image-size ;size in bytes as integer
(default 'guess))
(operating-system image-operating-system ;<operating-system>
(default #f))
(partitions image-partitions ;list of <partition>
(default '()))
(compression? image-compression? ;boolean
(default #t))
(volatile-root? image-volatile-root? ;boolean
(default #t))
(substitutable? image-substitutable? ;boolean
(default #t)))
@end lisp
This record contains the operating-system to instantiate. The
@code{format} field defines the image type and can be @code{efi-raw},
@code{qcow2} or @code{iso9660} for instance. In the future, it could be
extended to @code{docker} or other image types.
A new directory in the Guix sources is dedicated to images definition. For now
there are four files:
@itemize @bullet
@item @file{gnu/system/images/hurd.scm}
@item @file{gnu/system/images/pine64.scm}
@item @file{gnu/system/images/novena.scm}
@item @file{gnu/system/images/pinebook-pro.scm}
@end itemize
Let's have a look to @file{pine64.scm}. It contains the
@code{pine64-barebones-os} variable which is a minimal definition of an
operating-system dedicated to the @b{Pine A64 LTS} board.
@lisp
(define pine64-barebones-os
(operating-system
(host-name "vignemale")
(timezone "Europe/Paris")
(locale "en_US.utf8")
(bootloader (bootloader-configuration
(bootloader u-boot-pine64-lts-bootloader)
(target "/dev/vda")))
(initrd-modules '())
(kernel linux-libre-arm64-generic)
(file-systems (cons (file-system
(device (file-system-label "my-root"))
(mount-point "/")
(type "ext4"))
%base-file-systems))
(services (cons (service agetty-service-type
(agetty-configuration
(extra-options '("-L")) ; no carrier detect
(baud-rate "115200")
(term "vt100")
(tty "ttyS0")))
%base-services))))
@end lisp
The @code{kernel} and @code{bootloader} fields are pointing to packages
dedicated to this board.
Right below, the @code{pine64-image-type} variable is also defined.
@lisp
(define pine64-image-type
(image-type
(name 'pine64-raw)
(constructor (cut image-with-os arm64-disk-image <>))))
@end lisp
It's using a record we haven't talked about yet, the @code{image-type} record,
defined this way:
@lisp
(define-record-type* <image-type>
image-type make-image-type
image-type?
(name image-type-name) ;symbol
(constructor image-type-constructor)) ;<operating-system> -> <image>
@end lisp
The main purpose of this record is to associate a name to a procedure
transforming an @code{operating-system} to an image. To understand why
it is necessary, let's have a look to the command producing an image
from an @code{operating-system} configuration file:
@example
guix system image my-os.scm
@end example
This command expects an @code{operating-system} configuration but how
should we indicate that we want an image targeting a Pine64 board? We
need to provide an extra information, the @code{image-type}, by passing
the @code{--image-type} or @code{-t} flag, this way:
@example
guix system image --image-type=pine64-raw my-os.scm
@end example
This @code{image-type} parameter points to the @code{pine64-image-type}
defined above. Hence, the @code{operating-system} declared in
@code{my-os.scm} will be applied the @code{(cut image-with-os
arm64-disk-image <>)} procedure to turn it into an image.
The resulting image looks like:
@lisp
(image
(format 'disk-image)
(target "aarch64-linux-gnu")
(operating-system my-os)
(partitions
(list (partition
(inherit root-partition)
(offset root-offset)))))
@end lisp
which is the aggregation of the @code{operating-system} defined in
@code{my-os.scm} to the @code{arm64-disk-image} record.
But enough Scheme madness. What does this image API bring to the Guix user?
One can run:
@example
mathieu@@cervin:~$ guix system --list-image-types
The available image types are:
- pinebook-pro-raw
- pine64-raw
- novena-raw
- hurd-raw
- hurd-qcow2
- qcow2
- uncompressed-iso9660
- efi-raw
- arm64-raw
- arm32-raw
- iso9660
@end example
and by writing an @code{operating-system} file based on
@code{pine64-barebones-os}, you can customize your image to your
preferences in a file (@file{my-pine-os.scm}) like this:
@lisp
(use-modules (gnu services linux)
(gnu system images pine64))
(let ((base-os pine64-barebones-os))
(operating-system
(inherit base-os)
(timezone "America/Indiana/Indianapolis")
(services
(cons
(service earlyoom-service-type
(earlyoom-configuration
(prefer-regexp "icecat|chromium")))
(operating-system-user-services base-os)))))
@end lisp
run:
@example
guix system image --image-type=pine64-raw my-pine-os.scm
@end example
or,
@example
guix system image --image-type=hurd-raw my-hurd-os.scm
@end example
to get an image that can be written directly to a hard drive and booted
from.
Without changing anything to @code{my-hurd-os.scm}, calling:
@example
guix system image --image-type=hurd-qcow2 my-hurd-os.scm
@end example
will instead produce a Hurd QEMU image.
@node Connecting to Wireguard VPN
@section Connecting to Wireguard VPN