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Xen 3.0 Virtualization Interface Guide
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8. Devices

Virtual devices under Xen are provided by a split device driver architecture. The illusion of the virtual device is provided by two co-operating drivers: the frontend, which runs an the unprivileged domain and the backend, which runs in a domain with access to the real device hardware (often called a driver domain; in practice domain 0 usually fulfills this function).

The frontend driver appears to the unprivileged guest as if it were a real device, for instance a block or network device. It receives IO requests from its kernel as usual, however since it does not have access to the physical hardware of the system it must then issue requests to the backend. The backend driver is responsible for receiving these IO requests, verifying that they are safe and then issuing them to the real device hardware. The backend driver appears to its kernel as a normal user of in-kernel IO functionality. When the IO completes the backend notifies the frontend that the data is ready for use; the frontend is then able to report IO completion to its own kernel.

Frontend drivers are designed to be simple; most of the complexity is in the backend, which has responsibility for translating device addresses, verifying that requests are well-formed and do not violate isolation guarantees, etc.

Split drivers exchange requests and responses in shared memory, with an event channel for asynchronous notifications of activity. When the frontend driver comes up, it uses Xenstore to set up a shared memory frame and an interdomain event channel for communications with the backend. Once this connection is established, the two can communicate directly by placing requests / responses into shared memory and then sending notifications on the event channel. This separation of notification from data transfer allows message batching, and results in very efficient device access.

This chapter focuses on some individual split device interfaces available to Xen guests.

Xen 3.0 Virtualization Interface Guide
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  Published under the terms of the GNU General Public License Design by Interspire