[bug#41350] [PATCH 3/3] system: vm: Build vm-image using native qemu.

["Jan (janneke) Nieuwenhuizen" <janneke@gnu.org>]

Cross-building a vm-image used to be done using a cross-qemu, e.g, qemu-ARM.
That does not work for the Hurd, as there is no qemu-HURD.

This patch switches to cross building vm-images using a native qemu vm.

* gnu/system/vm.scm (expression->derivation-in-linux-vm): Run native
qemu-command; use native linux, initrd, bootloader-package and
bootloader-installer.
[preserve-target]: New helper to install cross-packages into the native vm.
* gnu/bootloader/grub.scm (eye-candy): Use native font.

Co-authored-by: Ludovic Courtès <ludo@gnu.org>
---
 gnu/bootloader/grub.scm |  4 ++--
 gnu/system/vm.scm       | 46 ++++++++++++++++++++++++++---------------
 2 files changed, 31 insertions(+), 19 deletions(-)

diff --git a/gnu/bootloader/grub.scm b/gnu/bootloader/grub.scm
index 8c5b5eac0c..842592ccc9 100644
--- a/gnu/bootloader/grub.scm
+++ b/gnu/bootloader/grub.scm
@@ -211,8 +211,8 @@ else
   set menu_color_highlight=white/blue
 fi~%"
                  #$setup-gfxterm-body
-                 #$(grub-root-search store-device font-file)
-                 #$(setup-gfxterm config font-file)
+                 #+(grub-root-search store-device font-file)
+                 #+(setup-gfxterm config font-file)
                  #$(grub-setup-io config)
 
                  #$(strip-mount-point store-mount-point image)
diff --git a/gnu/system/vm.scm b/gnu/system/vm.scm
index 97b0bf461b..aa094b0a06 100644
--- a/gnu/system/vm.scm
+++ b/gnu/system/vm.scm
@@ -5,6 +5,7 @@
 ;;; Copyright © 2017 Mathieu Othacehe <m.othacehe@gmail.com>
 ;;; Copyright © 2017 Marius Bakke <mbakke@fastmail.com>
 ;;; Copyright © 2018 Chris Marusich <cmmarusich@gmail.com>
+;;; Copyright © 2020 Jan (janneke) Nieuwenhuizen <janneke@gnu.org>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -180,19 +181,30 @@ made available under the /xchg CIFS share.
 SUBSTITUTABLE? determines whether the returned derivation should be marked as
 substitutable."
   (define user-builder
-    (program-file "builder-in-linux-vm" exp))
+    (scheme-file "builder-in-linux-vm" exp))
+
+  (define (preserve-target obj)
+    (if target
+        (with-parameters ((%current-target-system target))
+          obj)
+        obj))
 
   (define loader
-    ;; Invoke USER-BUILDER instead using 'primitive-load'.  The reason for
-    ;; this is to allow USER-BUILDER to dlopen stuff by using a full-featured
-    ;; Guile, which it couldn't do using the statically-linked guile used in
-    ;; the initrd.  See example at
+    ;; Instead of using 'primitive-load', evaluate USER-BUILDER in a
+    ;; full-featured Guile so it can use dlopen stuff, which it couldn't do
+    ;; using the statically-linked guile used in the initrd.  See example at
     ;; <https://lists.gnu.org/archive/html/guix-devel/2017-10/msg00233.html>.
     (program-file "linux-vm-loader"
-                  ;; Communicate USER-BUILDER's exit status via /xchg so that
-                  ;; the host can distinguish between success, failure, and
-                  ;; kernel panic.
-                  #~(let ((status (system* #$user-builder)))
+                  ;; When cross-compiling, USER-BUILDER refers to the target
+                  ;; (cross-compiled) system.  Preserve that, even though
+                  ;; LOADER itself is executed as a native program.
+                  #~(let ((status (system* #+(file-append (default-guile)
+                                                          "/bin/guile")
+                                           "--no-auto-compile"
+                                           #$(preserve-target user-builder))))
+                      ;; Communicate USER-BUILDER's exit status via /xchg so
+                      ;; that the host can distinguish between success,
+                      ;; failure, and kernel panic.
                       (call-with-output-file "/xchg/.exit-status"
                         (lambda (port)
                           (write status port)))
@@ -226,10 +238,10 @@ substitutable."
 
               (let* ((native-inputs
                       '#+(list qemu (canonical-package coreutils)))
-                     (linux   (string-append #$linux "/"
-                                             #$(system-linux-image-file-name)))
-                     (initrd  #$initrd)
-                     (loader  #$loader)
+                     (linux   (string-append #+linux "/"
+                                             #+(system-linux-image-file-name)))
+                     (initrd  #+initrd)
+                     (loader  #+loader)
                      (graphs  '#$(match references-graphs
                                    (((graph-files . _) ...) graph-files)
                                    (_ #f)))
@@ -245,7 +257,7 @@ substitutable."
                 (load-in-linux-vm loader
                                   #:output #$output
                                   #:linux linux #:initrd initrd
-                                  #:qemu (qemu-command target)
+                                  #:qemu (qemu-command)
                                   #:memory-size #$memory-size
                                   #:make-disk-image? #$make-disk-image?
                                   #:single-file-output? #$single-file-output?
@@ -348,7 +360,7 @@ system that is passed to 'populate-root-file-system'."
            (setlocale LC_ALL "en_US.utf8")
 
            (let ((inputs
-                  '#$(append (list parted e2fsprogs dosfstools)
+                  '#+(append (list parted e2fsprogs dosfstools)
                              (map canonical-package
                                   (list sed grep coreutils findutils gawk))))
 
@@ -425,12 +437,12 @@ system that is passed to 'populate-root-file-system'."
                                      #:partitions partitions
                                      #:grub-efi grub-efi
                                      #:bootloader-package
-                                     #$(bootloader-package bootloader)
+                                     #+(bootloader-package bootloader)
                                      #:bootcfg #$bootcfg-drv
                                      #:bootcfg-location
                                      #$(bootloader-configuration-file bootloader)
                                      #:bootloader-installer
-                                     #$(bootloader-installer bootloader)))))))
+                                     #+(bootloader-installer bootloader)))))))
    #:system system
    #:target target
    #:make-disk-image? #t
-- 
2.26.0