The Design and Implementation of the Anykernel and Rump Kernels

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By convention, file system device nodes are available in /dev. NetBSD does not
feature a device file system which dynamically creates device nodes based on the
drivers in the kernel. A standard installation of NetBSD relies on precreated device
nodes residing on persistent storage. We work around this issue in two ways.
First, during bootstrap, the rump kernel VFS faction generates a selection of common
device nodes such as /dev/zero. Second, we added support to various driver
attachments to create device driver nodes when the drivers are attached. These
adjustments avoid the requirement to have persistent storage mounted on /dev.
3.7.1 Extra-Terrestrial File System
The Extra-Terrestrial File System (etfs) interface provides a rump kernel with access
to files on the host. The etfs (manual page rump etfs.3 at A–20) interface is used to
register host file mappings with rumpfs. Fundamentally, the purpose of etfs is the
same as that of a hostfs available on most full system virtualization solutions. Unlike
a hostfs, which typically mounts a directory from the host, etfs is oriented towards
mapping individual files. The interface allows the registration of type and offset
translators for individual host files; a feature we will look at more closely below.
In addition, etfs only supports reading and writing files and cannot manipulate the
directory namespace on the host. This I/O-oriented approach avoids issues such as
how to map permissions on newly created hostfs files.
The mapping capability of etfs is hooked up to the lookup operation within rumpfs.
Recall, a lookup operation for a pathname will produce an in-memory file system
structure referencing the file behind that pathname. If the pathname under lookup
consists of a registered etfs key, the in-memory structure will be tagged so that
further I/O operations, i.e. read and write, will be directed to the backing file on
the host.