Normally, a minimal initrd is created automatically by the rcutorture
scripting.  But minimal really does mean "minimal", namely just a single
root directory with a single statically linked executable named "init":

$ size tools/testing/selftests/rcutorture/initrd/init
   text    data     bss     dec     hex filename
    328       0       8     336     150 tools/testing/selftests/rcutorture/initrd/init

Suppose you need to run some scripts, perhaps to monitor or control
some aspect of the rcutorture testing.  This will require a more fully
filled-out userspace, perhaps containing libraries, executables for
the shell and other utilities, and soforth.  In that case, place your
desired filesystem here:

	tools/testing/selftests/rcutorture/initrd

For example, your tools/testing/selftests/rcutorture/initrd/init might
be a script that does any needed mount operations and starts whatever
scripts need starting to properly monitor or control your testing.
The next rcutorture build will then incorporate this filesystem into
the kernel image that is passed to qemu.

Or maybe you need a real root filesystem for some reason, in which case
please read on!

The remainder of this document describes one way to create the
rcu-test-image file that contains the filesystem used by the guest-OS
kernel.  There are probably much better ways of doing this, and this
filesystem could no doubt be smaller.  It is probably also possible to
simply download an appropriate image from any number of places.

That said, here are the commands:

------------------------------------------------------------------------
dd if=/dev/zero of=rcu-test-image bs=400M count=1
mkfs.ext3 ./rcu-test-image
sudo mount -o loop ./rcu-test-image /mnt

# Replace "precise" below with your favorite Ubuntu release.
# Empirical evidence says this image will work for 64-bit, but...
# Note that debootstrap does take a few minutes to run.  Or longer.
sudo debootstrap --verbose --arch i386 precise /mnt http://archive.ubuntu.com/ubuntu
cat << '___EOF___' | sudo dd of=/mnt/etc/fstab
# UNCONFIGURED FSTAB FOR BASE SYSTEM
#
/dev/vda        /               ext3    defaults        1 1
dev             /dev            tmpfs   rw              0 0
tmpfs           /dev/shm        tmpfs   defaults        0 0
devpts          /dev/pts        devpts  gid=5,mode=620  0 0
sysfs           /sys            sysfs   defaults        0 0
proc            /proc           proc    defaults        0 0
___EOF___
sudo umount /mnt
------------------------------------------------------------------------


References:

	http://sripathikodi.blogspot.com/2010/02/creating-kvm-bootable-fedora-system.html
	https://help.ubuntu.com/community/KVM/CreateGuests
	https://help.ubuntu.com/community/JeOSVMBuilder
	http://wiki.libvirt.org/page/UbuntuKVMWalkthrough
	http://www.moe.co.uk/2011/01/07/pci_add_option_rom-failed-to-find-romfile-pxe-rtl8139-bin/ -- "apt-get install kvm-pxe"
	https://www.landley.net/writing/rootfs-howto.html
	https://en.wikipedia.org/wiki/Initrd
	https://en.wikipedia.org/wiki/Cpio
	http://wiki.libvirt.org/page/UbuntuKVMWalkthrough