Instructions for building a systemd images for Gentoo See: http://bencord0.wordpress.com/2013/09/02/systemd-stage3/ for details.

build_initramfs.sh

#!/bin/bash

dracut -f /root/initramfs -i /root/systemd.squashfs /root.squashfs 
chmod a+r /root/initramfs

build_kernel.sh

#!/bin/bash
MAKEOPTS="-j$(grep processor /proc/cpuinfo|wc -l)"

emerge -u sys-kernel/aufs-sources

cd /usr/src/linux

zcat /proc/config.gz > .config
make olddefconfig
make $MAKEOPTS targz-pkg

cd /
tar xzvf /usr/src/linux/linux*.tar.gz

depmod
cp /boot/vmlinuz* /root/vmlinuz

build_squashfs.sh

#!/bin/bash
rm -f /root/systemd.squashfs||true;
mksquashfs chroot /root/systemd.squashfs

build_stage3.sh

#!/bin/bash
TOPDIR=$(dirname $0)
cd "$TOPDIR"

EMERGE_FLAGS="--buildpkg --update --jobs"
DBUS_DEPS="sys-libs/glibc \
    sys-libs/cracklib \
    sys-libs/pam \
    sys-apps/shadow \
    sys-apps/baselayout"

rm -rf "chroot"
mkdir "chroot-prepare" "chroot"
tar xavpf stage-template.tar.gz -C chroot-prepare
tar xavpf stage-template.tar.gz -C chroot

# Stop when things go wrong
set -ex

# note: dbus's pkg_setup phase needs some files to exist in the chroot

# Building binary packages also installs compile-time dependencies
emerge $EMERGE_FLAGS --usepkg --config-root=chroot-prepare --root=chroot-prepare \
    --oneshot --nodeps $DBUS_DEPS
emerge $EMERGE_FLAGS --usepkg --config-root=chroot-prepare --root=chroot-prepare \
    --oneshot --nodeps sys-auth/pambase
emerge $EMERGE_FLAGS --usepkg --config-root=chroot-prepare --root=chroot-prepare \
    world

# Only install the runtime dependencies
emerge $EMERGE_FLAGS --usepkgonly --config-root=chroot --root=chroot \
    --oneshot --nodeps $DBUS_DEPS
emerge $EMERGE_FLAGS --usepkgonly --config-root=chroot --root=chroot \
    --oneshot --nodeps sys-auth/pambase
emerge $EMERGE_FLAGS --usepkgonly --config-root=chroot --root=chroot \
    world

# Blank out the default root password
sed -i -e '/root/ s/*//' chroot/etc/shadow

# Don't bother looking for other filesystems (esp. SWAP)
echo -n > chroot/etc/fstab

# Start networking on boot
ln -s 'chroot/usr/lib64/systemd/system/dhcpcd.service' \
    'chroot/etc/systemd/system/multi-user.target.wants/dhcpcd.service'

tar cJf /root/stage3-systemd.tar.xz -C chroot .

prepare_dracut.sh

#!/bin/bash
cd /usr/lib/dracut/modules.d


###########
# Console #
###########
mkdir -p 80console
cat << EOF > 80console/module-setup.sh
#!/bin/bash

check() {
    return 0
}

depends() {
    return 0
}

install() {
    inst "\$moddir/console-tty0.conf" /etc/cmdline.d/console-tty0.conf
    inst "\$moddir/console-ttyS0.conf" /etc/cmdline.d/console=ttyS0.conf
}
EOF
chmod +x 80console/module-setup.sh

echo 'console=tty0' > 80console/console-tty0.conf
echo 'console=ttyS0' > 80console/console-ttyS0.conf

##################################
# Rootfs = aufs(squashfs, tmpfs) #
##################################
mkdir -p 81squashedaufs-root
cat << EOF > 81squashedaufs-root/module-setup.sh
#!/bin/bash
check() {
    return 0
}

depends() {
    return 0
}

install() {
    inst_hook cmdline 81 "\$moddir/cmdline-squashedaufs-root.sh"
    inst_hook mount 81 "\$moddir/mount-squashedaufs-root.sh"
    inst_hook pre-pivot 81 "\$moddir/pre-pivot-squashedaufs-root.sh"
    inst "\$moddir/squashedaufs-root.conf" /etc/cmdline.d/squashedaufs-root.conf
}
EOF
chmod +x 81squashedaufs-root/module-setup.sh

cat << EOF > 81squashedaufs-root/cmdline-squashedaufs-root.sh
#!/bin/sh
case "\$root" in
    *.squashfs)
        wait_for_dev "\$root"
        rootok=1
        USING_SQUASHEDAUFS=1
        ;;
esac
EOF
chmod +x 81squashedaufs-root/cmdline-squashedaufs-root.sh

cat << EOF > 81squashedaufs-root/mount-squashedaufs-root.sh
#!/bin/bash
mount_squashfs_as_aufs()
{
    info "Creating a tmpfs for root"
    mkdir -p /tmproot
    mount -t tmpfs tmpfs /tmproot -o size=90%

    info "Mounting squashfs"
    mkdir -p /squashroot
    mount -t squashfs "\$root" /squashroot

    info "Unioning rootfs"
    mount -t aufs -o br:/tmproot:/squashroot none /sysroot
}

if [ -n USING_SQUASHEDAUFS ]
then
    mount_squashfs_as_aufs
fi
EOF
chmod +x 81squashedaufs-root/mount-squashedaufs-root.sh

cat << EOF >> 81squashedaufs-root/pre-pivot-squashedaufs-root.sh
#!/bin/bash
mkdir -p /sysroot/lib/modules
cp -r /lib/modules/* /sysroot/lib/modules/
EOF
chmod +x 81squashedaufs-root/pre-pivot-squashedaufs-root.sh

echo 'root=/root.squashfs' >> 81squashedaufs-root/squashedaufs-root.conf

stage-template.sh

#!/bin/bash

DIRS="
    dev
    etc/portage/package.keywords
    etc/portage/package.use
    home
    proc
    root
    sys
    usr/portage
    var/lib/portage
"

for dir in $DIRS; do
    mkdir -p stage-template/$dir
done

ln -sf /usr/portage/profiles/default/linux/amd64/13.0 stage-template/etc/make.profile

cat << EOF > stage-template/etc/portage/make.conf
ACCEPT_KEYWORDS="~amd64"
FEATURES="buildpkg parallel-fetch parallel-install"
MAKEOPTS="-j8"
USE="-bindist -consolekit systemd"
EOF

cat << EOF > stage-template/var/lib/portage/world
app-editors/vim
app-portage/eix
net-dns/bind-tools
net-misc/dhcpcd
sys-apps/dbus
sys-apps/iproute2
sys-apps/systemd
sys-boot/grub
sys-boot/os-prober
sys-fs/btrfs-progs
sys-fs/lvm2
sys-fs/squashfs-tools
sys-kernel/dracut
EOF

tar czf stage-template.tar.gz -C stage-template .

usage.sh

# Example usage in clean qemu environment
#
# $ qemu-kvm \
# $     -m 10G \
# $     -smp 6 \
# $     -kernel vmlinuz \
# $     -initrd initramfs \
# $     -net nic,model=virtio \
# $     -net user \
# $     -nographic \
# $     -append console=ttyS0

# Which is ofcourse, useless without me supplying 
# the vmlinuz and initramfs files.
# Creation of the initramfs is sill a manual task
# It does some things atypical to a standard distro initramfs.
#   - Only a kernel and initramfs are supplied to qemu, there
#     is no stateful disk image.
#   - The root= kernel cmdline is set inside the initramfs,
#     not supplied to qemu's "-append".
#   - The VM uses a lot of RAM. All writes are directed
#     towards an RAM backed AUFS rootfs.
#   - QEMU User networking provides a crippeled network
#     environment, sufficient enough for TCP to download
#     the portage tree and distfiles.
#   - No other special networking needs to be made. Qemu can be run
#     as a non-privilaged user.
#   - While the kernel is fairly standard
#     (sys-kernel/aufs-sources), however, network drivers and other
#     config is taylored for my environment.
#   - Kernel modules insude the initramfs are coupled to
#     the kernel version. (In theory, the initramfs could be
#     bundled into the kernel too)
#   - The initramfs is at least as big as any generated
#     tarball since the initramfs will effectively perform
#     a stage3 install during the boot process. It is a neat
#     chicken/egg problem that requires the stage3 to build the
#     initramfs, and the qemu/kernel/initramfs to (cleanly) build
#     the stage tarball.
#   - My custom dracut module is not documented (or described) here.

# Once the VM has booted, root login is permitted (without password)

# Set SYNC and GENTOO_MIRRORS variables in /etc/portage/make.conf first.
emerge --sync
eix-update

# Enable ssh, not sure why the ebuild didn't set these directories properly
chown root /var/empty
chmod 755 /var/empty
systemctl start sshd

# Run /usr/bin/passwd to set a password and enable remote logins
# (optional) passwd

# For some reason, binutils postinstall actions are not run properly.
# Manually make the symlinks, orphans will eventually be replaced
ln -sf /usr/x86_64-pc-linux-gnu/binutils-bin/2.23.2/* /usr/bin
emerge binutils
source /etc/profile

emerge -uDNvj dev-vcs/git world

git clone https://gist.github.com/6407310.git
(cd /; patch -p0 -l < /root/6407310/user.eclass.patch)

cd 6407310
bash stage-template.sh
bash build_stage3.sh
bash build_squashfs.sh
bash build_kernel.sh
bash prepare_dracut.sh
bash build_initramfs.sh

user.eclass.patch

--- /usr/portage/eclass/user.eclass.orig     2012-06-22 20:31:13.000000000 +0100
+++ /usr/portage/eclass/user.eclass          2013-09-01 16:03:48.000000000 +0100
@@ -92,9 +92,13 @@
                grep "${key}:\*:" /etc/${db}
                ;;
        *)
-               # ignore output if nscd doesn't exist, or we're not running as root
-               nscd -i "${db}" 2>/dev/null
-               getent "${db}" "${key}"
+               if [[ "${ROOT}" != "/" ]] ; then
+                       chroot "${ROOT}" getent "${db}" "${key}"
+               else
+                       # ignore output if nscd doesn't exist, or we're not running as root
+                       nscd -i "${db}" 2>/dev/null
+                       getent "${db}" "${key}"
+               fi
                ;;
        esac
 }
@@ -247,7 +251,7 @@
                ;;

        *)
-               useradd -r "${opts[@]}" "${euser}" || die
+               useradd -R "${ROOT}" -r "${opts[@]}" "${euser}" || die
                ;;
        esac

@@ -340,7 +344,7 @@
                        opts="-g ${egid}"
                fi
                # We specify -r so that we get a GID in the system range from login.defs
-               groupadd -r ${opts} "${egroup}" || die
+               groupadd -R "${ROOT}" -r ${opts} "${egroup}" || die
                ;;
        esac
 }