Facilitating the Development of Soft Devices
Andrew Warfield, Keir Fraser, Steven Hand and Tim Deegan
Proc. USENIX ʼ05, pp. 379-382, April 2005.

Device-level interfaces in operating systems present a very useful cut-point for researchers to experiment with new ideas. By virtualizing these interfaces, developers can create soft devices, which are used in the same way as normal hardware devices, but provide extra functionality in software. Recent years have shown this approach to be of considerable interest: a few examples of block device extension include the addition of intrusion detection systems to disk interfaces, the development of “semantically smart” disks, and that of time-travel block devices. Other devices, such as network interfaces, have similarly been extended.

Working at the device interface allows an examination of the functional separation between hardware and software: researchers can simulate new features as if they were properties of the device itself. As simple examples, block or network device interfaces might be extended to compress or encrypt data before it is written to disk or transmitted. Alternatively, it may be desirable to prototype entirely new devices in software, bound to existing interfaces, for instance a content-addressable disk.

Unfortunately, researchers face a challenge in extending devices in this manner. Implementors must typically modify an existing operating system to add the new functionality, often by creating OS-specific pseudo-devices. This requirement means learning the OS source and writing scaffolding code to intercept events. Moreover, where new functionality must be developed in-kernel it is difficult to debug and crashes are not contained. Finally, these low-level developments are difficult to share and maintain across systems, as they will be specific to the OS, or even specific version thereof, that it has been developed within.

This paper presents a solution to the problems associated with developing soft devices by extending the existing device interface in Xen. Xen is a virtual machine monitor (VMM) for the IA32 architecture that paravirtualizes hardware: Rather than attempting to present a fully virtualized hardware interface to each OS in a Xen environment, guest OSes are modified to use a simple, narrow and idealized view of hardware. Soft devices take advantage of these narrow interface to capture and transform block requests, network packets, and USB messages.

As an initial example of this approach, we have implemented a block tap, which is an interface to facilitate the development of soft devices for block device access. The block tap allows soft devices to be constructed as user-space applications in an entirely isolated virtual machine. This strong isolation from the remainder of the system allows a single soft device to work with any OS and hardware available on Xen, and allows developers to work with high-level languages and debuggers. While our approach aims to facilitate development it still provides a high level of performance, sustaining 50MB/s read throughput for disk requests in our experiments.