[bug#65842,1/2] image: Introduce the mbr-hybrid-raw image type.

diff --git a/doc/guix.texi b/doc/guix.texi
index 339dcb2a41..22135254ea 100644
--- a/doc/guix.texi
+++ b/doc/guix.texi
@@ -40912,8 +40912,8 @@  Invoking guix system
 @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{mbr-raw}.  When its value is @code{iso9660}, the
-@option{--label} option can be used to specify a volume ID with
+defaults to @code{mbr-hybrid-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
 make it volatile instead.  When using @code{image}, the bootloader
@@ -40931,8 +40931,8 @@  Invoking guix system
                    -bios $(guix build ovmf)/share/firmware/ovmf_x64.bin
 @end example
 
-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
+When using the @code{mbr-hybrid-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:
 
@@ -41069,7 +41069,7 @@  Invoking guix system
 For the @code{image} action, create an image with given @var{type}.
 
 When this option is omitted, @command{guix system} uses the
-@code{mbr-raw} image type.
+@code{mbr-hybrid-raw} image type.
 
 @cindex ISO-9660 format
 @cindex CD image format
@@ -45277,7 +45277,7 @@  System Images
 
 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{mbr-raw}, @code{iso9660} and
+different image types, such as @code{mbr-hybrid-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
@@ -45541,8 +45541,24 @@  Instantiate an Image
 system image)} module provides the following @code{image} definition
 variables.
 
+@defvar mbr-disk-image
+An MBR disk-image composed of a single ROOT partition.  The ROOT
+partition starts at a 1@tie{}MiB offset so that the bootloader can
+install itself in the post-MBR gap.
+@end defvar
+
+@defvar mbr-hybrid-disk-image
+An MBR disk-image composed of two partitions: a 64 bits ESP partition
+and a ROOT boot partition.  The ESP partition starts at a 1@tie{}MiB
+offset so that a BIOS compatible bootloader can install itself in the
+post-MBR gap.  The image can be used by @code{x86_64} and @code{i686}
+machines supporting only legacy BIOS booting.  The ESP partition ensures
+that it can also be used by newer machines relying on UEFI booting,
+hence the @emph{hybrid} denomination.
+@end defvar
+
 @defvar efi-disk-image
-A MBR disk-image composed of two partitions: a 64 bits ESP partition and
+A GPT disk-image composed of two partitions: a 64 bits ESP partition and
 a ROOT boot partition.  This image can be used on most @code{x86_64} and
 @code{i686} machines, supporting BIOS or UEFI booting.
 @end defvar
@@ -45633,6 +45649,10 @@  image-type Reference
 Build an image based on the @code{mbr-disk-image} image.
 @end defvar
 
+@defvar mbr-hybrid-raw-image-type
+Build an image based on the @code{mbr-hybrid-disk-image} image.
+@end defvar
+
 @defvar efi-raw-image-type
 Build an image based on the @code{efi-disk-image} image.
 @end defvar
diff --git a/gnu/ci.scm b/gnu/ci.scm
index 520ac28110..279dd4d910 100644
--- a/gnu/ci.scm
+++ b/gnu/ci.scm
@@ -268,7 +268,7 @@  (define* (image-jobs store system
     (if (member system %guix-system-supported-systems)
         `(,(image->job store
                        (image
-                        (inherit efi-disk-image)
+                        (inherit mbr-hybrid-disk-image)
                         (operating-system installation-os))
                        #:name "usb-image"
                        #:system system)
diff --git a/gnu/system/image.scm b/gnu/system/image.scm
index 5b8da2f896..b1b928b222 100644
--- a/gnu/system/image.scm
+++ b/gnu/system/image.scm
@@ -77,6 +77,7 @@  (define-module (gnu system image)
             root-partition
 
             mbr-disk-image
+            mbr-hybrid-disk-image
             efi-disk-image
             iso9660-image
             docker-image
@@ -86,6 +87,7 @@  (define-module (gnu system image)
 
             image-with-os
             mbr-raw-image-type
+            mbr-hybrid-raw-image-type
             efi-raw-image-type
             efi32-raw-image-type
             qcow2-image-type
@@ -156,6 +158,13 @@  (define mbr-disk-image
            (inherit root-partition)
            (offset root-offset))))))
 
+(define mbr-hybrid-disk-image
+  (image-without-os
+   (format 'disk-image)
+   (partition-table-type 'mbr)
+   (partitions
+    (list esp-partition root-partition))))
+
 (define efi-disk-image
   (image-without-os
    (format 'disk-image)
@@ -217,6 +226,11 @@  (define mbr-raw-image-type
    (name 'mbr-raw)
    (constructor (cut image-with-os mbr-disk-image <>))))
 
+(define mbr-hybrid-raw-image-type
+  (image-type
+   (name 'mbr-hybrid-raw)
+   (constructor (cut image-with-os mbr-hybrid-disk-image <>))))
+
 (define efi-raw-image-type
   (image-type
    (name 'efi-raw)
diff --git a/guix/scripts/system.scm b/guix/scripts/system.scm
index ec331809ef..547387d5e1 100644
--- a/guix/scripts/system.scm
+++ b/guix/scripts/system.scm
@@ -1169,7 +1169,7 @@  (define %default-options
     (debug . 0)
     (verbosity . #f)                              ;default
     (validate-reconfigure . ,ensure-forward-reconfigure)
-    (image-type . mbr-raw)
+    (image-type . mbr-hybrid-raw)
     (image-size . guess)
     (install-bootloader? . #t)
     (label . #f)