The Design and Implementation of the Anykernel and Rump Kernels

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Figure 3.30: File system server. The request from the microkernel client is
transported by the host kernel to the rump kernel running providing the kernel file
system driver. Although only system calls are illustrated, page faults created by the
client may be handled by the server as well.
3.11 Microkernel Servers: Case Study with File Servers
In this section we investigate using rump kernels as microkernel style servers for file
systems. Our key motivation is to prevent a malfunctioning file system driver from
damaging the host kernel by isolating it in a userspace server.
The NetBSD framework for implementing file servers in userspace is called puffs [53].
We use puffs to attach the rump kernel file server to the host’s file system namespace.
Conceptually, after the file system has been mounted, the service works as follows: a
file system request is transported from the host kernel to the userspace server using
puffs. The server makes a local call into the rump kernel to service the request.
When servicing the request is complete, the response is returned to the host kernel
using puffs. The architecture of this solution is presented in Figure 3.30. It is worth
noting that a userlevel application is not the only possible consumer. Any VFS user,
such as an NFS server running in the host kernel, is a valid consumer in this model.