The Design and Implementation of the Anykernel and Rump Kernels

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Our implementation is different from the above. Instead of doing transport and
network layer processing in the rump kernel, we observe that regardless of what the
guest does, processing will be done by the host. At best, we would need to undo what
the guest did so that we can feed the payload data to the host’s sockets interface.
Instead of using the TCP/IP protocol suite in the rump kernel, we redefine the inet
domain, and attach our implementation at the protocol switch layer [114]. We call
this new implementation sockin to reflect it being socket inet. The attachment
to the kernel is illustrated in Figure 3.23. Attaching at the domain level means
communication from the kernel’s socket layer is done with usrreq’s, which in turn
map to the host socket API in a very straightforward manner. For example, for
PRU_ATTACH we call socket(), for PRU_BIND we call bind(), for PRU_CONNECT
we call connect(), and so forth. The whole implementation is 500 lines of code
(including whitespace and comments), making it 1/20th of the size of Slirp.
Since sockin attaches as the Internet domain, it is mutually exclusive with the regular
TCP/IP protocol suite. Furthermore, since the interface layer is excluded, the sockin
approach is not suitable for scenarios which require full TCP/IP processing within
the virtual kernel, e.g. debugging the TCP/IP stack. In such cases one of the other
two networking models should be used. This choice may be made individually for
each rump kernel instance.
3.9.2 Disk Driver
A disk block device driver provides storage medium access and is instrumental to
the operation of disk-based file systems. The main interface is simple: a request
instructs the driver to read or write a given number of sectors at a given offset.
The disk driver queues the request and returns. The request is handled in an order
according to a set policy, e.g. the disk head elevator. The request must be handled in
a timely manner, since during the period that the disk driver is handling the request