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1.4.3. Physically shared devices

Certain hardware platforms allow virtualized guests to directly access various hardware devices and components. This process in virtualization is known as passthrough. Passthrough is known as device assignment in some of the KVM documentation and the KVM code.
PCI passthrough
The KVM hypervisor supports attaching PCI devices on the host system to virtualized guests. PCI passthrough allows guests to have exclusive access to PCI devices for a range of tasks. PCI passthrough allows PCI devices to appear and behave as if they were physically attached to the guest operating system.
Almost all PCI and PCI Express devices that support passthrough, except for graphics cards, can be directly attached to virtualized guests with PCI passthrough.
SR-IOV (Single Root I/O Virtualization) is a standard for a type of PCI passthrough which natively shares a single device to multiple guests.
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 new PCI functions:
  • Virtualization-PF
  • Virtualization VF
For more information on SR-IOV, refer to Chapter 13, SR-IOV.
N_Port ID Virtualization (NPIV) is a function available with some Fibre Channel devices. NPIV shares a single physical N_Port as multiple N_Port IDs. NPIV provides similar functionality for Host Bus Adaptors (HBAs) that SR-IOV provides for network interfaces. With NPIV, virtualized guests can be provided with a virtual Fibre Channel initiator to Storage Area Networks (SANs).
NPIV can provide high density virtualized environments with enterprise-level storage solutions.
For more information on NPIV, refer to Chapter 29, N_Port ID Virtualization (NPIV).

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