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Over the last six years, Galois has been developing the Haskell Lightweight Virtual Machine, or HaLVM, a lightweight virtual machine that runs directly on the Xen hypervisor. The HaLVM's design is based on a notion of minimalism: Authors of HaLVM domains include only those libraries and features they require, allowing the HaLVM to have a very small initial resource footprint that scales in a very obvious fashion. In doing so, we hope to combine the minimalism and flexibility of a small kernel with an extended set of libraries with the simplicity and reliability of the strongly typed, high-level language Haskell.
While initially designed for running operating system design experiments, the HaLVM has grown over time to be a suitable platform for writing simple network appliances with a very narrow resource footprint. This work has been enabled by other Galois projects: a TCP-compliant network stack written in Haskell, and a fairly-complete file system written in Haskell. In the end, because we use Haskell and provide these libraries at the Haskell level, programmers can create complex software structures quickly, easily, and with the added assurance that the Haskell type system provides.
In this talk, we will provide an overview of the HaLVM and its design principles - pointing out where the HaLVM shines and where it is weak - and continue with some of our experiences using it over the last six years.