Single Root I/O Virtualization or SR-IOV for short is a PCI Express (PCIe) specification, that allows PCI Express adapters to be shared between multiple operating systems. Similar to micro-partitioning, several guest operating systems (LPARs) get shares of a physical PCIe adapter. The big advantage, in terms of performance, is that the hardware virtualization is completely performed by the physical PCIe adapter. A virtual I/O server, as required for other virtualization technologies, is not necessary for SR-IOV. The LPARs can address and use the PCIe adapter directly. This saves processor resources in particular and also reduces I/O latency.

Physical functions (PF) and virtual functions (VF) are introduced to support virtualization. Physical functions support all the features of a PCIe adapter, in particular the configuration of the physical device (chip) and I/O. Virtual functions are lightweight versions of the physical functions, that do not allow any configuration of the physical device (chip), but only support I/O. This is shown schematically in figure 7.17.