image: Introduce the mbr-hybrid-raw image type.

Until 209204e23b and
d57cab7641, the default image type used by "guix
system image" was an MBR image with an ESP partition.

Having both an MBR image and an ESP partition is handy because the image will
boot on most x86 based systems using legacy BIOS and/or UEFI.

We now have a distinction between MBR images and EFI images. Introduce a new
MBR hybrid image type and default to it to restore the default behaviour.

This also fixes the images section of (gnu ci) that was trying to install a
BIOS bootloader on an EFI, GPT image and failing to do so.

Signed-off-by: Mathieu Othacehe <othacehe@gnu.org>

doc/guix.texi

@@ -40982,8 +40982,8 @@ 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{mbr-raw}.  When its value is @code{iso9660}, the
+defaults to @code{mbr-hybrid-raw}.  When its value is @code{iso9660},
-@option{--label} option can be used to specify a volume ID with
+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
@@ -41001,8 +41001,8 @@ 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{mbr-raw} image type, a raw disk image is produced;
+When using the @code{mbr-hybrid-raw} image type, a raw disk image is
-it can be copied as is to a USB stick, for instance.  Assuming
+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:
 
@@ -41139,7 +41139,7 @@ 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{mbr-raw} image type.
+@code{mbr-hybrid-raw} image type.
 
 @cindex ISO-9660 format
 @cindex CD image format
@@ -45347,7 +45347,7 @@ 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{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
@@ -45611,8 +45611,24 @@ from them to simplify the @code{image} definition.  The @code{(gnu
 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
@@ -45703,6 +45719,10 @@ system image)} and the @code{(gnu system images @dots{})} modules.
 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

gnu/ci.scm

@@ -268,7 +268,7 @@ (define MiB
     (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)

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)

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)