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.\" Copyright (c) 2011 Bryan Venteicher
.\" All rights reserved.
.\"
.\" Redistribution and use in source and binary forms, with or without
.\" modification, are permitted provided that the following conditions
.\" are met:
.\" 1. Redistributions of source code must retain the above copyright
.\" notice, this list of conditions and the following disclaimer.
.\" 2. Redistributions in binary form must reproduce the above copyright
.\" notice, this list of conditions and the following disclaimer in the
.\" documentation and/or other materials provided with the distribution.
.\"
.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
.\" ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
.\" SUCH DAMAGE.
.\"
.Dd April 4, 2026
.Dt VIRTIO 4
.Os
.Sh NAME
.Nm virtio
.Nd VirtIO Device Support
.Sh SYNOPSIS
To compile VirtIO device support into the kernel, place the following lines
in your kernel configuration file:
.Bd -ragged -offset indent
.Cd "device virtio"
.Cd "device virtio_pci"
.Ed
.Pp
Alternatively, to load VirtIO support as modules at boot time, place the
following lines in
.Xr loader.conf 5 :
.Bd -literal -offset indent
virtio_load="YES"
virtio_pci_load="YES"
.Ed
.Sh 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.
.Pp
VirtIO defines an interface for efficient I/O between the hypervisor and VM.
The
.Nm
module provides a shared memory transport called a virtqueue.
The
.Sy virtio_pci
device driver represents an emulated PCI device that the hypervisor makes
available to the VM.
This device provides the probing, configuration, and
interrupt notifications needed to interact with the hypervisor.
.Fx
supports the following VirtIO devices:
.Bl -hang -offset indent -width xxxxxxxx
.It Sy Ethernet
An emulated Ethernet device is provided by the
.Xr vtnet 4
device driver.
.It Sy Block
An emulated disk controller is provided by the
.Xr virtio_blk 4
device driver.
.It Sy Console
Provided by the
.Xr virtio_console 4
driver.
.It Sy Entropy
Provided by the
.Xr virtio_random 4
driver.
.It Sy Balloon
A pseudo-device to allow the VM to release memory back to the hypervisor is
provided by the
.Xr virtio_balloon 4
device driver.
.It Sy GPU
Graphics support is provided by the
.Xr virtio_gpu 4
device driver.
.It Sy SCSI
An emulated SCSI HBA is provided by the
.Xr virtio_scsi 4
device driver.
.El
.Sh LOADER TUNABLES
Tunables can be set at the
.Xr loader 8
prompt before booting the kernel or stored in
.Xr loader.conf 5 .
.Bl -tag -width "hw.virtio.pci.transitional"
.It Va hw.virtio.pci.disable_msix
If set to 1, disables MSI-X.
The default value is 0.
.It Va hw.virtio.pci.transitional
For a transitional
.Nm
device, this tunable specifies whether to negotiate
modern mode and use the modern
.Nm
driver
.Pq 1
or to negotiate legacy mode and
use the legacy
.Nm
driver
.Pq 0 .
The default value is 1.
.El
.Sh SEE ALSO
.Xr virtio_balloon 4 ,
.Xr virtio_blk 4 ,
.Xr virtio_console 4 ,
.Xr virtio_gpu 4 ,
.Xr virtio_random 4 ,
.Xr virtio_scsi 4 ,
.Xr vtnet 4
.Sh HISTORY
Support for VirtIO first appeared in
.Fx 9.0 .
.Sh AUTHORS
.An -nosplit
.Fx
support for VirtIO was first added by
.An Bryan Venteicher Aq Mt bryanv@FreeBSD.org .
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