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virtio(4) [freebsd man page]

VIRTIO(4)						   BSD Kernel Interfaces Manual 						 VIRTIO(4)

virtio -- VirtIO Device Support SYNOPSIS
To compile VirtIO device support into the kernel, place the following lines in your kernel configuration file: device virtio device virtio_pci Alternatively, to load VirtIO support as modules at boot time, place the following lines in loader.conf(5): virtio_load="YES" virtio_pci_load="YES" DESCRIPTION
VirtIO is a specification for para-virtualized I/O in a virtual machine (VM). Traditionally, the hypervisor emulated real devices such as an Ethernet interface or disk controller to provide the VM with I/O. This emulation is often inefficient. VirtIO defines an interface for efficient I/O between the hypervisor and VM. The virtio module provides a shared memory transport called a virtqueue. The virtio_pci device driver represents an emulated PCI device that the hypervisor makes available to the VM. This device pro- vides the probing, configuration, and interrupt notifications needed to interact with the hypervisor. FreeBSD supports the following VirtIO devices: Ethernet An emulated Ethernet device is provided by the vtnet(4) device driver. Block An emulated disk controller is provided by the virtio_blk(4) device driver. SCSI An emulated SCSI HBA is provided by the virtio_scsi(4) device driver. Balloon A pseudo-device to allow the VM to release memory back to the hypervisor is provided by the virtio_balloon(4) device driver. SEE ALSO
virtio_balloon(4), virtio_blk(4), virtio_console(4), virtio_scsi(4), vtnet(4) HISTORY
Support for VirtIO first appeared in FreeBSD 9.0. AUTHORS
FreeBSD support for VirtIO was first added by Bryan Venteicher <>. BSD
January 22, 2012 BSD

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XEN(4)							   BSD Kernel Interfaces Manual 						    XEN(4)

xen -- Xen Hypervisor Guest (DomU) Support SYNOPSIS
To compile para-virtualized (PV) Xen guest support into an i386 kernel, place the following lines in your kernel configuration file: options PAE options XEN nooptions NATIVE To compile hardware-assisted virtualization (HVM) Xen guest support with para-virtualized drivers into an amd64 kernel, place the following lines in your kernel configuration file: options XENHVM device xenpci DESCRIPTION
The Xen Hypervisor allows multiple virtual machines to be run on a single computer system. When first released, Xen required that i386 ker- nels be compiled "para-virtualized" as the x86 instruction set was not fully virtualizable. Primarily, para-virtualization modifies the vir- tual memory system to use hypervisor calls (hypercalls) rather than direct hardware instructions to modify the TLB, although para-virtualized device drivers were also required to access resources such as virtual network interfaces and disk devices. With later instruction set extensions from AMD and Intel to support fully virtualizable instructions, unmodified virtual memory systems can also be supported; this is referred to as hardware-assisted virtualization (HVM). HVM configurations may either rely on transparently emu- lated hardware peripherals, or para-virtualized drivers, which are aware of virtualization, and hence able to optimize certain behaviors to improve performance or semantics. FreeBSD supports a fully para-virtualized (PV) kernel on the i386 architecture using options XEN and nooptions NATIVE; currently, this requires use of a PAE kernel, enabled via options PAE. FreeBSD supports hardware-assisted virtualization (HVM) on both the i386 and amd64 kernels; however, PV device drivers with an HVM kernel are only supported on the amd64 architecture, and require options XENHVM and device xenpci. Para-virtualized device drivers are required in order to support certain functionality, such as processing management requests, returning idle physical memory pages to the hypervisor, etc. Xen DomU device drivers Xen para-virtualized drivers are automatically added to the kernel if a PV kernel is compiled using options XEN; for HVM environments, options XENHVM and device xenpci are required. The follow drivers are supported: balloon Allow physical memory pages to be returned to the hypervisor as a result of manual tuning or automatic policy. blkback Exports local block devices or files to other Xen domains where they can then be imported via blkfront. blkfront Import block devices from other Xen domains as local block devices, to be used for file systems, swap, etc. console Export the low-level system console via the Xen console service. control Process management operations from Domain 0, including power off, reboot, suspend, crash, and halt requests. evtchn Expose Xen events via the /dev/xen/evtchn special device. netback Export local network interfaces to other Xen domains where they can be imported via netfront. netfront Import network interfaces from other Xen domains as local network interfaces, which may be used for IPv4, IPv6, etc. pcifront Allow physical PCI devices to be passed through into a PV domain. xenpci Represents the Xen PCI device, an emulated PCI device that is exposed to HVM domains. This device allows detection of the Xen hypervisor, and provides interrupt and shared memory services required to interact with the hypervisor. Performance considerations In general, PV drivers will perform better than emulated hardware, and are the recommended configuration for HVM installations. Using a hypervisor introduces a second layer of scheduling that may limit the effectiveness of certain FreeBSD scheduling optimisations. Among these is adaptive locking, which is no longer able to determine whether a thread holding a lock is in execution. It is recommended that adaptive locking be disabled when using Xen: options NO_ADAPTIVE_MUTEXES options NO_ADAPTIVE_RWLOCKS options NO_ADAPTIVE_SX SEE ALSO
pae(4) HISTORY
Support for xen first appeared in FreeBSD 8.1. AUTHORS
FreeBSD support for Xen was first added by Kip Macy <> and Doug Rabson <>. Further refinements were made by Justin Gibbs <>, Adrian Chadd <>, and Colin Percival <>. This manual page was written by Robert Watson <>. BUGS
FreeBSD is only able to run as a Xen guest (DomU) and not as a Xen host (Dom0). A fully para-virtualized (PV) kernel is only supported on i386, and not amd64. Para-virtualized drivers under hardware-assisted virtualization (HVM) kernel are only supported on amd64, not i386. As of this release, Xen PV DomU support is not heavily tested; instability has been reported during VM migration of PV kernels. Certain PV driver features, such as the balloon driver, are under-exercised. BSD
December 17, 2010 BSD
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