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(___()'`; Rusty's Remarkably Unreliable Guide to Lguest
/, /` - or, A Young Coder's Illustrated Hypervisor
Lguest is designed to be a minimal hypervisor for the Linux kernel, for
Linux developers and users to experiment with virtualization with the
minimum of complexity. Nonetheless, it should have sufficient
features to make it useful for specific tasks, and, of course, you are
encouraged to fork and enhance it (see drivers/lguest/README).
- Kernel module which runs in a normal kernel.
- Simple I/O model for communication.
- Simple program to create new guests.
- Logo contains cute puppies:
Developer features:
- Fun to hack on.
- No ABI: being tied to a specific kernel anyway, you can change anything.
- Many opportunities for improvement or feature implementation.
Running Lguest:
- The easiest way to run lguest is to use same kernel as guest and host.
You can configure them differently, but usually it's easiest not to.
You will need to configure your kernel with the following options:
"General setup":
"Prompt for development and/or incomplete code/drivers" = Y
"Processor type and features":
"Paravirtualized guest support" = Y
"Lguest guest support" = Y
"High Memory Support" = off/4GB
"Alignment value to which kernel should be aligned" = 0x100000
"Device Drivers":
"Block devices"
"Virtio block driver (EXPERIMENTAL)" = M/Y
"Network device support"
"Universal TUN/TAP device driver support" = M/Y
"Virtio network driver (EXPERIMENTAL)" = M/Y
"Linux hypervisor example code" = M/Y
- A tool called "lguest" is available in this directory: type "make"
to build it. If you didn't build your kernel in-tree, use "make
- Create or find a root disk image. There are several useful ones
around, such as the xm-test tiny root image at
For more serious work, I usually use a distribution ISO image and
install it under qemu, then make multiple copies:
dd if=/dev/zero of=rootfile bs=1M count=2048
qemu -cdrom image.iso -hda rootfile -net user -net nic -boot d
Make sure that you install a getty on /dev/hvc0 if you want to log in on the
- "modprobe lg" if you built it as a module.
- Run an lguest as root:
Documentation/lguest/lguest 64 vmlinux --tunnet= --block=rootfile root=/dev/vda
64: the amount of memory to use, in MB.
vmlinux: the kernel image found in the top of your build directory. You
can also use a standard bzImage.
--tunnet= configures a "tap" device for networking with this
IP address.
--block=rootfile: a file or block device which becomes /dev/vda
inside the guest.
root=/dev/vda: this (and anything else on the command line) are
kernel boot parameters.
- Configuring networking. I usually have the host masquerade, using
"iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE" and "echo 1 >
/proc/sys/net/ipv4/ip_forward". In this example, I would configure
eth0 inside the guest at
Another method is to bridge the tap device to an external interface
using --tunnet=bridge:<bridgename>, and perhaps run dhcp on the guest
to obtain an IP address. The bridge needs to be configured first:
this option simply adds the tap interface to it.
A simple example on my system:
ifconfig eth0
brctl addbr lg0
ifconfig lg0 up
brctl addif lg0 eth0
dhclient lg0
Then use --tunnet=bridge:lg0 when launching the guest.
See for general information
on how to get bridging working.
There is a helpful mailing list at
Good luck!
Rusty Russell