lguest — a minimal x86 hypervisor for Linux

October 25, 2009

~6k lines of C code, and you have a x86 hypervisor in the Linux kernel!

lguest is a very minimal x86 hypervisor for Linux, very easy to set up, and due to its very small code base, it can be fun to hack/tweak.

Besides its small code base, it’s very well documented, thus ‘studying’ the lguest code is probably the best way to start learning a few things about virtualization and hypervisors.

Actually, the Makefile in drivers/lguest/ can be used to ‘extract’ the comments from the lguest code, in a very organized way, which makes it even easier to understand how lguest works(just cd to the directory and type make Beer, or make Puppy, if you want some ascii puppies : )

lguest requires some kernel configuration. Except for lguest itself, you must include some virtio modules, particularly virtio_blk and virtio_net, if you need networking inside the guest.

Afterwards, you must compile the Documentation/lguest/lguest.c, which you’ll run in userspace every time you want to ‘launch’ a new guest.

In order to run a new guest, you just run the lguest launher you just compiled, with some parameters, like the amount of memory the guest should have, the kernel image(usually the same image, that the host runs) and/or the initrd needed, a rootfs(you can either download a minimal rootfs for testing, or create one using qemu), and the virtual block device(/dev/vda), where the rootfs will be mounted inside the guest.
i.e.
lguest 64m vmlinux --initrd=initrd.img --block=rootfile root=/dev/vda
That's it! You have a guest running.

Of course lguest lacks features, included in other hypervisors/virtualization solutions, but that's the point. A small 'experimental' hypervisor, easy to understand and maybe tweak. ;)

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