maint: The 'release' target builds a VM image.

* gnu/system/examples/vm-image.tmpl: New file.
* Makefile.am (GUIXSD_VM_SYSTEMS, GUIXSD_VM_IMAGE_BASE,
GUIXSD_VM_IMAGE_SIZE): New variables.
(release): Add logic to build a VM image.
(EXAMPLES): Add 'gnu/system/examples/vm-image.tmpl'.
* doc/guix.texi (Running GuixSD in a VM, Installing GuixSD in a VM): Mention the
pre-built VM image.

Makefile.am

@@ -5,6 +5,7 @@
 # Copyright © 2016 Mathieu Lirzin <mthl@gnu.org>
 # Copyright © 2016, 2017 Mark H Weaver <mhw@netris.org>
 # Copyright © 2017 Mathieu Othacehe <m.othacehe@gmail.com>
+# Copyright © 2017 Leo Famulari <leo@famulari.name>
 #
 # This file is part of GNU Guix.
 #
@@ -224,7 +225,8 @@ AUX_FILES =						\
 EXAMPLES =					\
   gnu/system/examples/bare-bones.tmpl		\
   gnu/system/examples/desktop.tmpl		\
-  gnu/system/examples/lightweight-desktop.tmpl
+  gnu/system/examples/lightweight-desktop.tmpl	\
+  gnu/system/examples/vm-image.tmpl
 
 GOBJECTS = $(MODULES:%.scm=%.go) guix/config.go $(dist_noinst_DATA:%.scm=%.go)
 
@@ -572,12 +574,21 @@ BINARY_TARBALLS =							\
 # Systems supported by GuixSD.
 GUIXSD_SUPPORTED_SYSTEMS ?= x86_64-linux i686-linux
 
+# Systems for which we build GuixSD VMs.
+GUIXSD_VM_SYSTEMS ?= x86_64-linux
+
 # Prefix of the GuixSD installation image file name.
 GUIXSD_IMAGE_BASE = guixsd-usb-install-$(PACKAGE_VERSION)
 
+# Prefix of the GuixSD VM image file name.
+GUIXSD_VM_IMAGE_BASE = guixsd-vm-image-$(PACKAGE_VERSION)
+
 # Size of the installation image (for x86_64 typically).
 GUIXSD_INSTALLATION_IMAGE_SIZE ?= 950MiB
 
+# Size of the VM image (for x86_64 typically).
+GUIXSD_VM_IMAGE_SIZE ?= 2GiB
+
 # The release process works in several phases:
 #
 #   0. We assume the developer created a 'vX.Y' tag.
@@ -632,6 +643,20 @@ release: dist
 	  mv "$(releasedir)/$(GUIXSD_IMAGE_BASE).$$system.xz.tmp"			\
 	     "$(releasedir)/$(GUIXSD_IMAGE_BASE).$$system.xz" ;				\
 	done
+	for system in $(GUIXSD_VM_SYSTEMS) ; do						\
+	  image=`$(top_builddir)/pre-inst-env						\
+	    guix system vm-image							\
+	    --system=$$system								\
+	    --image-size=$(GUIXSD_VM_IMAGE_SIZE)					\
+	    gnu/system/examples/vm-image.tmpl` ;					\
+	  if [ ! -f "$$image" ] ; then							\
+	    echo "failed to produced GuixSD VM image for $$system" >&2 ;		\
+	    exit 1 ;									\
+	  fi ;										\
+	  xz < "$$image" > "$(releasedir)/$(GUIXSD_VM_IMAGE_BASE).$$system.xz.tmp" ;	\
+	  mv "$(releasedir)/$(GUIXSD_VM_IMAGE_BASE).$$system.xz.tmp"			\
+	     "$(releasedir)/$(GUIXSD_VM_IMAGE_BASE).$$system.xz" ;			\
+	done
 	@echo
 	@echo "Congratulations!  All the release files are now in $(releasedir)."
 	@echo

doc/guix.texi

@@ -7634,8 +7634,11 @@ good.
 @subsection Installing GuixSD in a Virtual Machine
 
 @cindex virtual machine, GuixSD installation
-If you'd like to install GuixSD in a virtual machine (VM) rather than on
+@cindex virtual private server (VPS)
-your beloved machine, this section is for you.
+@cindex VPS (virtual private server)
+If you'd like to install GuixSD in a virtual machine (VM) or on a
+virtual private server (VPS) rather than on your beloved machine, this
+section is for you.
 
 To boot a @uref{http://qemu.org/,QEMU} VM for installing GuixSD in a
 disk image, follow these steps:
@@ -15693,17 +15696,21 @@ example graph.
 @subsection Running GuixSD in a Virtual Machine
 
 @cindex virtual machine
-One way to run GuixSD in a virtual machine (VM) is to build a GuixSD
+To run GuixSD in a virtual machine (VM), one can either use the
-virtual machine image using @command{guix system vm-image}
+pre-built GuixSD VM image distributed at
-(@pxref{Invoking guix system}).  The returned image is in qcow2 format,
+@indicateurl{ftp://alpha.gnu.org/guix/guixsd-vm-image-@value{VERSION}.@var{system}.tar.xz}
-which the @uref{http://qemu.org/, QEMU emulator} can efficiently use.
+, or build their own virtual machine image using @command{guix system
+vm-image} (@pxref{Invoking guix system}).  The returned image is in
+qcow2 format, which the @uref{http://qemu.org/, QEMU emulator} can
+efficiently use.
 
 @cindex QEMU
-To run the image in QEMU, copy it out of the store (@pxref{The Store})
+If you built your own image, you must copy it out of the store
-and give yourself permission to write to the copy.  When invoking QEMU,
+(@pxref{The Store}) and give yourself permission to write to the copy
-you must choose a system emulator that is suitable for your hardware
+before you can use it.  When invoking QEMU, you must choose a system
-platform.  Here is a minimal QEMU invocation that will boot the result
+emulator that is suitable for your hardware platform.  Here is a minimal
-of @command{guix system vm-image} on x86_64 hardware:
+QEMU invocation that will boot the result of @command{guix system
+vm-image} on x86_64 hardware:
 
 @example
 $ qemu-system-x86_64 \

gnu/system/examples/vm-image.tmpl

@@ -0,0 +1,53 @@
+;;; This is an operating system configuration template for a "bare-bones" setup,
+;;; suitable for booting in a virtualized environment, including virtual private
+;;; servers (VPS).
+
+(use-modules (gnu))
+(use-package-modules bootloaders disk nvi)
+
+(define vm-image-motd (plain-file "motd" "
+This is the GNU system.  Welcome!
+
+This instance of GuixSD is a bare-bones template for virtualized environments.
+
+You will probably want to do these things first if you booted in a virtual
+private server (VPS):
+
+* Set a password for 'root'.
+* Set up networking.
+* Expand the root partition to fill the space available by 0) deleting and
+recreating the partition with fdisk, 1) reloading the partition table with
+partprobe, and then 2) resizing the filesystem with resize2fs.\n"))
+
+(operating-system
+  (host-name "gnu")
+  (timezone "Etc/UTC")
+  (locale "en_US.utf8")
+
+  ;; Assuming /dev/sdX is the target hard disk, and "my-root" is
+  ;; the label of the target root file system.
+  (bootloader (grub-configuration (device "/dev/sda")
+                                  (terminal-outputs '(console))))
+  (file-systems (cons (file-system
+                        (device "my-root")
+                        (title 'label)
+                        (mount-point "/")
+                        (type "ext4"))
+                      %base-file-systems))
+
+  ;; This is where user accounts are specified.  The "root"
+  ;; account is implicit, and is initially created with the
+  ;; empty password.
+  (users %base-user-accounts)
+
+  ;; Globally-installed packages.
+  (packages (cons* nvi fdisk
+                   grub   ; mostly so xrefs to its manual work
+                   parted ; partprobe
+                   %base-packages))
+
+  (services (modify-services %base-services
+              (login-service-type config =>
+                                  (login-configuration
+                                    (inherit config)
+                                    (motd vm-image-motd))))))