Chapter 13. SR-IOV

Developed by the PCI-SIG (PCI Special Interest Group), the Single Root I/O Virtualization (SR-IOV) specification is a standard for a type of PCI device assignment that can share a single device to multiple virtual machines. SR-IOV improves device performance for virtual machines.

SR-IOV enables a Single Root Function (for example, a single Ethernet port), to appear as multiple, separate, physical devices. A physical device with SR-IOV capabilities can be configured to appear in the PCI configuration space as multiple functions. Each device has its own configuration space complete with Base Address Registers (BARs).

SR-IOV uses two PCI functions:

The hypervisor can map one or more Virtual Functions to a virtual machine. The Virtual Function's configuration space is then mapped to the configuration space presented to the guest.

Each Virtual Function can only be mapped to a single guest at a time, as Virtual Functions require real hardware resources. A virtual machine can have multiple Virtual Functions. A Virtual Function appears as a network card in the same way as a normal network card would appear to an operating system.

The SR-IOV drivers are implemented in the kernel. The core implementation is contained in the PCI subsystem, but there must also be driver support for both the Physical Function (PF) and Virtual Function (VF) devices. An SR-IOV capable device can allocate VFs from a PF. The VFs appear as PCI devices which are backed on the physical PCI device by resources such as queues and register sets.

Virtual Functions have near-native performance and provide better performance than paravirtualized drivers and emulated access. Virtual Functions provide data protection between virtual machines on the same physical server as the data is managed and controlled by the hardware.

These features allow for increased virtual machine density on hosts within a data center.

SR-IOV is better able to utilize the bandwidth of devices with multiple guests.