Mick Jordan
Sun
JavaGuest
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Many server-based commercial environments run application suites that
are entirely implemented in Java, or in languages that compile to Java
bytecode. The Java applications are executed by a Java virtual
machine (JVM) that is hosted on a traditional operating system,
typically some variant of Unix. While Unix implementations continue to
evolve and improve, Unix was primarily designed to execute C
applications and is not an ideal match for the requirements of the
Java platform. While it possible in principle to modify the
operating system, this is not easy, nor it is possible to exploit the
potential offered by the safety properties of the Java language, which
does not require the address-space protection needed by unsafe
languages such as C.

Until recently the effort involved in developing a new operating
system more suited to hosting the Java platform was prohibitive. The
advent of para-virtualized hypervisors, e.g. Xen [BDF+03] changes this
situation by dramatically lowering the barrier to entry. Not only does
the hypervisor provide a portable and idealized interface to the
hardware, but also the ability to concurrently run standard operating
systems allows some facilities to be off-loaded completely.

While some of the benefits could be realized by porting an existing
C-based JVM to the hypervisor API, [D06], [Ammons07] the full
potential of a dynamically optimizing JVM environment can only be
realized with an all- Java approach. Accordingly, the JavaGuest
project [JavaGuest07] is based on Maxwell [Maxwell07], a new research
JVM under development at Sun Labs, that is written entirely in Java
and based on a meta-circular architecture. While there is already much
less native code in Maxwell than a traditional JVM, the JavaGuest
project has a particular goal of absolutely minimal C code, thus
allowing the Java compiler to optimize code down to the hypervisor API
layer.

The initial JavaGuest prototype is currently based on an enhanced
version of the MiniOS example guest OS that is provided in the
Xen distribution. MiniOS is augmented by a small layer that interfaces
with the Maxwell JVM. MiniOS has been modified for SMP support
(multiple VCPUs) and we have also added two custom splitdevice
drivers. The first driver provides just those I/O facilities required
by the file system classes in the Java platform, thereby off-loading
all file system access to a sibling guest OS. The second driver
provides support for debugging the Maxwell JVM using the Maxwell
Inspector, a powerful GUI-based tool that provides traditional
assembly-level debugging in conjunction with convenient visual access
to the JVM meta-data. At the time of writing, the system is capable of
running simple programs and our goal is to scale up to running a
standard application server. In contrast to file-system access, the
network stack will be implemented in Java and not offloaded.  In the
future, we plan to progressively replace as much as possible of the
MiniOS C code base with a Java implementation and exploit the
optimization possibilities inherent in the single-language
architecture.