image: Add mbr-raw-image-type and use by default.

* gnu/system/image.scm (mbr-disk-image, mbr-raw-image-type): New variables.
(qcow2-image-type): Inherit mbr-disk-image.
* guix/scripts/system.scm (%default-options): Use mbr-raw-image-type by
default.
* gnu/tests/install.scm (run-install): Use mbr-raw in the tests.
* doc/guix-cookbook.texi (Guix System Image API): Update the list of image
types.
* doc/guix.texi (Invoking guix system, System Images, image-type Reference):
Add mbr-raw and switch documented default to it.

doc/guix-cookbook.texi

@@ -2019,17 +2019,23 @@ One can run:
 mathieu@@cervin:~$ guix system --list-image-types
 The available image types are:
 
+   - unmatched-raw
+   - rock64-raw
    - pinebook-pro-raw
    - pine64-raw
    - novena-raw
    - hurd-raw
    - hurd-qcow2
    - qcow2
-   - uncompressed-iso9660
-   - efi-raw
-   - arm64-raw
-   - arm32-raw
    - iso9660
+   - uncompressed-iso9660
+   - tarball
+   - efi-raw
+   - mbr-raw
+   - docker
+   - wsl2
+   - raw-with-offset
+   - efi32-raw
 @end example
 
 and by writing an @code{operating-system} file based on

doc/guix.texi

@@ -40884,7 +40884,7 @@ QEMU monitor and the VM.
 @cindex image, creating disk images
 The @code{image} command can produce various image types.  The image
 type can be selected using the @option{--image-type} option.  It
-defaults to @code{efi-raw}.  When its value is @code{iso9660}, the
+defaults to @code{mbr-raw}.  When its value is @code{iso9660}, the
 @option{--label} option can be used to specify a volume ID with
 @code{image}.  By default, the root file system of a disk image is
 mounted non-volatile; the @option{--volatile} option can be provided to
@@ -40903,7 +40903,7 @@ qemu-system-x86_64 -enable-kvm -hda /tmp/my-image.qcow2 -m 1000 \
                    -bios $(guix build ovmf)/share/firmware/ovmf_x64.bin
 @end example
 
-When using the @code{efi-raw} image type, a raw disk image is produced;
+When using the @code{mbr-raw} image type, a raw disk image is produced;
 it can be copied as is to a USB stick, for instance.  Assuming
 @code{/dev/sdc} is the device corresponding to a USB stick, one can copy
 the image to it using the following command:
@@ -41041,7 +41041,7 @@ of the image.
 For the @code{image} action, create an image with given @var{type}.
 
 When this option is omitted, @command{guix system} uses the
-@code{efi-raw} image type.
+@code{mbr-raw} image type.
 
 @cindex ISO-9660 format
 @cindex CD image format
@@ -45193,7 +45193,7 @@ then directly boot from it, without any kind of installation procedure.
 
 The @command{guix system image} command is able to turn an operating
 system definition into a bootable image.  This command supports
-different image types, such as @code{efi-raw}, @code{iso9660} and
+different image types, such as @code{mbr-raw}, @code{iso9660} and
 @code{docker}.  Any modern @code{x86_64} machine will probably be able
 to boot from an @code{iso9660} image.  However, there are a few machines
 out there that require specific image types.  Those machines, in general
@@ -45545,6 +45545,10 @@ record.
 There are several @code{image-type} records provided by the @code{(gnu
 system image)} and the @code{(gnu system images @dots{})} modules.
 
+@defvar mbr-raw-image-type
+Build an image based on the @code{mbr-disk-image} image.
+@end defvar
+
 @defvar efi-raw-image-type
 Build an image based on the @code{efi-disk-image} image.
 @end defvar
@@ -45554,7 +45558,7 @@ Build an image based on the @code{efi32-disk-image} image.
 @end defvar
 
 @defvar qcow2-image-type
-Build an image based on the @code{efi-disk-image} image but with the
+Build an image based on the @code{mbr-disk-image} image but with the
 @code{compressed-qcow2} image format.
 @end defvar
 
@@ -45625,7 +45629,7 @@ wsl -d Guix
 
 So, if we get back to the @code{guix system image} command taking an
 @code{operating-system} declaration as argument.  By default, the
-@code{efi-raw-image-type} is used to turn the provided
+@code{mbr-raw-image-type} is used to turn the provided
 @code{operating-system} into an actual bootable image.
 
 To use a different @code{image-type}, the @code{--image-type} option can

gnu/system/image.scm

@@ -76,6 +76,7 @@
             esp32-partition
             root-partition
 
+            mbr-disk-image
             efi-disk-image
             iso9660-image
             docker-image
@@ -84,6 +85,7 @@
             raw-with-offset-disk-image
 
             image-with-os
+            mbr-raw-image-type
             efi-raw-image-type
             efi32-raw-image-type
             qcow2-image-type
@@ -145,6 +147,15 @@ parent image record."
    (flags '(boot))
    (initializer (gexp initialize-root-partition))))
 
+(define mbr-disk-image
+  (image-without-os
+   (format 'disk-image)
+   (partition-table-type 'mbr)
+   (partitions
+    (list (partition
+           (inherit root-partition)
+           (offset root-offset))))))
+
 (define efi-disk-image
   (image-without-os
    (format 'disk-image)
@@ -201,6 +212,11 @@ set to the given OS."
    (inherit base-image)
    (operating-system os)))
 
+(define mbr-raw-image-type
+  (image-type
+   (name 'mbr-raw)
+   (constructor (cut image-with-os mbr-disk-image <>))))
+
 (define efi-raw-image-type
   (image-type
    (name 'efi-raw)
@@ -216,8 +232,7 @@ set to the given OS."
    (name 'qcow2)
    (constructor (cut image-with-os
                  (image
-                  (inherit efi-disk-image)
-                  (partition-table-type 'mbr)
+                  (inherit mbr-disk-image)
                   (name 'image.qcow2)
                   (format 'compressed-qcow2))
                  <>))))

gnu/tests/install.scm

@@ -229,7 +229,7 @@ reboot\n")
                                                 (gnu installer tests)
                                                 (guix combinators))))
                       (uefi-support? #f)
-                      (installation-image-type 'efi-raw)
+                      (installation-image-type 'mbr-raw)
                       (install-size 'guess)
                       (target-size (* 2200 MiB))
                       (number-of-disks 1))
@@ -291,7 +291,7 @@ such as for RAID systems."
                        '("-bios" #$uefi-firmware)
                        '())
                  #$@(cond
-                     ((eq? 'efi-raw installation-image-type)
+                     ((eq? 'mbr-raw installation-image-type)
                       #~("-drive"
                          ,(string-append "file=" #$image
                                          ",if=virtio,readonly")))

guix/scripts/system.scm

@@ -1169,7 +1169,7 @@ Some ACTIONS support additional ARGS.\n"))
     (debug . 0)
     (verbosity . #f)                              ;default
     (validate-reconfigure . ,ensure-forward-reconfigure)
-    (image-type . efi-raw)
+    (image-type . mbr-raw)
     (image-size . guess)
     (install-bootloader? . #t)
     (label . #f)