Device drivers are an important part of any operating system—without them, the kernel (and thus the applications) can't communicate with physical hardware attached to the system.
Most full virtualization solutions provide emulated forms of simple devices. The emulated devices are typically chosen to be common hardware, so it is likely that drivers exist already for any given guest. Examples of hardware emulated include simple IDE hard disks and NE2000 network interfaces. This is a reasonable solution in cases where the guest cannot be modified, and is used by Xen in HVM domains where unmodified guests are run.
Paravirtualized guests, however, need to be modified in order to run anyway. As such, the requirement for the virtual environment to use existing drivers disappears. Making guest kernel authors write a lot of code, however, would not be a very good design decision, and so Xen devices must be simple to implement. They should also be fast; if they are not, they have no advantage over emulated devices.
The Xen approach is to provide abstract devices that implement a high-level interface that corresponds to a particular device category. Rather than providing a SCSI device and an IDE device, Xen provides an abstract block device. This supports only two operations: read and write a block. This is implemented in a way that closely corresponds to the POSIX readv and writev calls, allowing operations to be grouped in a single request (which allows I/O reordering in the Domain 0 kernel or the controller to be used effectively). The network interface is slightly more complicated, but still relatively easy for a guest to implement.
6.1 The Split Driver Model
Supporting the range of hardware available for a commodity PC would be a daunting task for Xen. Fortunately, most of the required hardware is already supported by the guest in Domain 0. If Xen can reuse this support, it gets a large amount of hardware compatibility for free.
In addition, it is fairly common for an operating system to already provide some multiplexing services. The purpose of an operating system (as opposed to running applications directly on the hardware) is to provide an abstraction of the real hardware. One of the features of this abstraction in a modern OS is that applications are, in general, not aware of each other. Two applications can use the same physical disk, network interface, or sound device, without worrying about others. By piggy-backing on this capability, Xen can avoid writing a lot of new and untested code.
This multiplexing capability is quite important. Some devices on high-end systems, particularly mainframes, are virtualization-aware. They can be partitioned in the firmware, and each running operating system can interact with them directly. For consumer-grade hardware, however, this is not common. Most consumer-level devices assume a single user, and require the running operating system to perform any required multiplexing. In a virtualized environment, device access must be multiplexed before it is handed over to the operating system.
As discussed earlier, the hypervisor provides a simple mechanism for communicating between domains: shared memory. This is used by device drivers to establish a connection between the two components. The I/O ring mechanism, described later in this chapter, is typically used for this.
One important thing to note about Xen devices is that they are not really part of Xen. The hypervisor provides the mechanisms for device discovery and moving data between domains; the drives are split across a pair of guest domains. Typically, this pair is Domain 0 and another guest, although it is also possible to use a dedicated driver domain instead of Domain 0. The interface is specified by Xen; however, the actual implementation is left up to the domains.
Figure 6.1 shows the structure of a typical split device driver. The front and back ends are isolated from each other in separate domains, and communicate solely by mechanisms provided by Xen. The most common of these is the I/O ring, built on top of the shared memory mechanism provided by Xen.
Figure 6.1 The composition of a split device driver
Shared memory rings alone would require a lot of polling, which is not always particularly efficient, although it can be fast where there is pending data in a large percentage of the polled cases. This need is eliminated by the Xen event mechanism, which allows asynchronous notifications. This is used to tell the back end that a request is waiting to be processed, or to tell a front end that there is a response waiting. Handling and delivering events is discussed in the next chapter.
The final part of the jigsaw puzzle is the XenStore. This is a simple hierarchical structure that is shared between domains. Unlike the grant tables, the interface is fairly high-level. One of the main uses for it is device discovery. In this rôle, it is analogous to the device tree provided by OpenFirmware, although it has additional uses. The guest in Domain 0 exports a tree containing the devices available to each unprivileged domain. This is used for the initial device discovery phase. The tree is traversed by the guest that wants to run front-end drivers, and any interesting devices are configured. The one exception to this is the console driver. It is anticipated that the console device is needed (or, at the very least, wanted) early on during the boot process, so it is advertised via the start info page.
The XenStore itself is implemented as a split device. The location of the page used to communicate is given as a machine frame number in the start info page. This is slightly different to other devices, in that the page is made available to the guest before the system starts, rather than being exported via the grant table mechanism and advertised in the XenStore.