One of the goals for the 4.2 release is for xl to have feature parity with xm for the most important functions. But along the way, we’ve also been adding a number of improvements to the interface as well. One of the ways in which xl has changed and improved the interface is in passing through pci devices directly to VMs.

A Basic device pass-through review

As you may know, Xen has for several years had the ability to “pass through” a pci device directly to a guest, allowing that guest to control the driver. This has several applications, including driver domains and increased performance for graphics or networking.

To pass through a device, you need to find out its BDF (Bus, Device, Function). A BDF now consists of three or four numbers in this format: DDDD:bb:dd.f, where: