The Design and Implementation of the Anykernel and Rump Kernels

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Figure 2.1: Rump kernel hierarchy. The desired drivers dictate the required
components. The factions are orthogonal and depend only on the rump kernel base.
The rump kernel base depends purely on the hypercall layer.
We use the term component to describe a functional unit for a rump kernel. For
example, a file system driver is a component. A rump kernel is constructed by linking
together the desired set of components, either at compile-time or at run-time. A
loose similarity exists between kernel modules and the rump kernel approach: code
is compiled once per target architecture, and a linker is used to determine runtime
features. For a given driver to function properly, the rump kernel must be linked with
the right set of dependencies. For example, the NFS component requires both the
file system and networking factions, but in contrast the tmpfs component requires
only the file system faction.
User interfaces are used by applications to request services from rump kernels. Any
dependencies induced by user interfaces are optional, as we will illustrate next. Consider
Unix-style device driver access. Access is most commonly done through file
system nodes in /dev, with the relevant user interfaces being open and read/write