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VIRTIO_MMIO(4)        DragonFly Kernel Interfaces Manual        VIRTIO_MMIO(4)

NAME
     virtio_mmio - VirtIO memory-mapped I/O transport driver

SYNOPSIS
     To compile this driver into the kernel, place the following lines in your
     kernel configuration file:

           device virtio
           device virtio_mmio

     Alternatively, to load the driver as a module at boot time, place the
     following line in loader.conf(5):

           virtio_mmio_load="YES"

DESCRIPTION
     The virtio_mmio driver provides the memory-mapped I/O (MMIO) transport
     for virtio(4), as an alternative to the PCI transport used on emulated
     PCI buses.  Instead of being discovered on a bus, each transport instance
     occupies a fixed window of physical address space and a single interrupt,
     on top of which the usual front-end drivers such as vtnet(4),
     virtio_blk(4) and virtio_scsi(4) attach.

     The driver implements the legacy (version 1) virtio-mmio register layout.
     It does not use FDT or ACPI for enumeration.  Instead, the address,
     interrupt and optional unit number of each device are described by the
     kernel environment (kenv(1)) variables documented below, and the
     corresponding virtio_mmio children are attached to the nexus pseudo-bus.
     This makes the driver suitable for lightweight virtual machine monitors,
     such as QEMU -M microvm and Firecracker, that present virtio devices over
     MMIO rather than PCI.

LOADER TUNABLES
     The following tunables describe the virtio-mmio devices to attach.  They
     can be set at the loader(8) prompt before booting the kernel or stored in
     loader.conf(5).

     hw.virtio.mmio.device               Describes the first virtio-mmio
                                         device.

     hw.virtio.mmio.device_N             Describes an additional virtio-mmio
                                         device, where N is a decimal number
                                         starting at 1.  The variables are
                                         consulted in order and enumeration
                                         stops at the first value of N that is
                                         not set, so the numbers must be
                                         contiguous.

     Each variable holds a device description of the form:

           size@baseaddr:irq[:unit]

     The fields are:

     size          Size of the device's MMIO register window in bytes.  A
                   single-letter binary suffix (K, M, G, T, P or E,
                   case-insensitive) may be appended, each multiplying the
                   value by 1024.

     baseaddr      Physical base address of the register window.

     irq           Interrupt number (GSI) delivered by the device.

     unit          Optional device unit number.  If omitted, the next
                   available unit is assigned automatically.

     The size, baseaddr, irq and unit fields accept a leading `0x' for
     hexadecimal or `0' for octal, as in strtoul(3).  This is the same syntax
     used by the Linux virtio_mmio.device kernel command-line parameter that
     many virtual machine monitors emit.

EXAMPLES
     Attach a single virtio-mmio device with a 512-byte register window at
     physical address 0xd0000000 on interrupt 5, by adding to loader.conf(5):

           hw.virtio.mmio.device="0x200@0xd0000000:5"

     Attach two devices:

           hw.virtio.mmio.device="512@0xd0000000:5"
           hw.virtio.mmio.device_1="512@0xd0000200:6"

SEE ALSO
     kenv(1), virtio(4), virtio_blk(4), virtio_scsi(4), vtnet(4),
     loader.conf(5), loader(8)

HISTORY
     The virtio_mmio driver first appeared in DragonFly 6.5.

AUTHORS
     The virtio_mmio driver was adapted for DragonFly from the FreeBSD virtio-
     mmio transport by Antonio Huete Jimenez <tuxillo@quantumachine.net>.  The
     FreeBSD command-line and kenv device-discovery support it is based on was
     written by
     Colin Percival <cperciva@FreeBSD.org>.

DragonFly 6.5-DEVELOPMENT        July 4, 2026        DragonFly 6.5-DEVELOPMENT