The Design and Implementation of the Anykernel and Rump Kernels

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boothowto variable. Since there is no virtual CPU to schedule before bootstrap,
no entry/exit wrappers are necessary. These bootstrap interfaces provided as nongenerated
prototypes in sys/rump/include/rump/rump.h.
3.7 Rump Kernel Root File System
Full operating systems require a root file system with persistent storage for files such
as /bin/ls and /etc/passwd. A rump kernel does not inherently require such files.
This relaxed requirement is because a rump kernel does not have a default userspace
and because client binaries are executed outside of the rump kernel. However,
specific drivers or clients may require file system support for example to open a
device, load firmware or access a file system image. In some cases, such as for
firmware files and file system images, it is likely that the backing storage for the
data to be accessed resides on the host.
We explicitly want to avoid mandating the association of persistent storage with a
rump kernel because the storage image requires setup and maintenance and would
hinder especially one-time invocations. It is not impossible to store the file system
hierarchy and data required by a specific rump kernel instance on persistent storage.
We are merely saying it is not required.
A file system driver called rumpfs was written. It is implemented in the source module
sys/rump/librump/rumpvfs/rumpfs.c. Like tmpfs, rumpfs is an in-memory
file system. Unlike tmpfs, which is as fast and as complete as possible, rumpfs is as
lightweight as possible. Most rumpfs operations have only simple implementations
and support for advanced features such as rename and NFS export has been omitted.
If these features are desired, an instance of tmpfs can be mounted within the
rump kernel when required. The lightweight implementation of rumpfs makes the
compiled size 3.5 times smaller than that of tmpfs.