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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 December 19, 2025
+.Dt VTNET 4
+.Os
+.Sh NAME
+.Nm vtnet
+.Nd VirtIO Ethernet driver
+.Sh SYNOPSIS
+To compile this driver into the kernel,
+place the following lines in your
+kernel configuration file:
+.Bd -ragged -offset indent
+.Cd "device vtnet"
+.Ed
+.Pp
+Alternatively, to load the driver as a
+module at boot time, place the following line in
+.Xr loader.conf 5 :
+.Bd -literal -offset indent
+if_vtnet_load="YES"
+.Ed
+.Sh DESCRIPTION
+The
+.Nm
+device driver provides support for VirtIO Ethernet devices.
+.Pp
+If the hypervisor advertises the appropriate features, the
+.Nm
+driver supports TCP/UDP checksum offload for both transmit and receive,
+TCP segmentation offload (TSO), TCP large receive offload (LRO),
+hardware VLAN tag stripping/insertion features, a multicast hash filter,
+as well as Jumbo Frames (up to 9216 bytes), which can be
+configured via the interface MTU setting.
+.Pp
+Two flavors of TCP LRO are supported:
+Hardware TCP LRO, which is performed by the host providing TCP segments larger
+than the MTU to the guest, and Software TCP LRO, which is performed by the
+network stack of the guest processing TCP segments in an optimized way.
+Only one flavour of TCP LRO should be used.
+Since hardware TCP LRO can have bad interactions with IP forwarding and
+software TCP LRO mitigates several drawbacks of hardware TCP LRO, the default
+setting is to disable hardware TCP LRO.
+See the loader tunable
+.Va hw.vtnet. Ns Ar X Ns Va .lro_disable .
+.Pp
+TCP/UDP receive checksum offload cannot be configured independently for IPv4
+and IPv6.
+Selecting an MTU larger than 1500 bytes with the
+.Xr ifconfig 8
+utility configures the adapter to receive and transmit Jumbo Frames.
+.Pp
+For more information on configuring this device, see
+.Xr ifconfig 8 .
+.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 "xxxxxx"
+.It Va hw.vtnet.csum_disable
+.It Va hw.vtnet. Ns Ar X Ns Va .csum_disable
+This tunable disables receive and transmit checksum offloading for TCP and
+UDP.
+This also implies that TCP segmentation offloading and large receive offload
+are disabled.
+The default value is 0.
+.It Va hw.vtnet.fixup_needs_csum
+.It Va hw.vtnet. Ns Ar X Ns Va .fixup_needs_csum
+This tunable enforces the calculation of a valid TCP or UDP checksum for
+packets received with 
+.Dv VIRTIO_NET_HDR_F_NEEDS_CSUM
+being set in the
+.Va flags
+field of the structure
+.Vt struct virtio_net_hdr .
+It also marks the checksum as being correct in the mbuf packet header.
+The default value is 0.
+This tunable is deprecated and will be removed in
+.Fx 16 .
+.It Va hw.vtnet.tso_disable
+.It Va hw.vtnet. Ns Ar X Ns Va .tso_disable
+This tunable disables TCP segmentation offloading.
+The default value is 0.
+.It Va hw.vtnet.lro_disable
+.It Va hw.vtnet. Ns Ar X Ns Va .lro_disable
+This tunable disables hardware TCP LRO.
+The default value is 1.
+.It Va hw.vtnet.mq_disable
+.It Va hw.vtnet. Ns Ar X Ns Va .mq_disable
+This tunable disables multiqueue.
+The default value is 0.
+.It Va hw.vtnet.mq_max_pairs
+.It Va hw.vtnet. Ns Ar X Ns Va .mq_max_pairs
+This tunable sets the maximum number of transmit and receive queue pairs.
+Multiple queues are only supported when the Multiqueue feature is negotiated.
+This driver supports a maximum of 8 queue pairs.
+The number of queue pairs used is the lesser of the maximum supported by the
+driver and the hypervisor, the number of CPUs present in the guest, and this
+tunable if not zero.
+The default value is 0.
+.It Va hw.vtnet.tso_maxlen
+.It Va hw.vtnet. Ns Ar X Ns Va .tso_maxlen
+This tunable sets the TSO burst limit.
+The default value is 65535.
+.It Va hw.vtnet.rx_process_limit
+.It Va hw.vtnet. Ns Ar X Ns Va .rx_process_limit
+This tunable sets the number of RX segments processed in one pass.
+The default value is 1024.
+.It Va hw.vtnet.lro_entry_count
+.It Va hw.vtnet. Ns Ar X Ns Va .lro_entry_count
+This tunable sets the software TCP LRO entry count.
+The default value is 128, the minimum value is 8.
+.It Va hw.vtnet.lro_mbufq_depth
+.It Va hw.vtnet. Ns Ar X Ns Va .lro_mbufq_depth
+This tunable sets the depth of the software TCP LRO mbuf queue.
+The default value is 0.
+.It Va hw.vtnet.altq_disable
+This tunable disables ALTQ support, allowing the use of multiqueue instead.
+This option applies to all interfaces.
+The default value is 0.
+.El
+.Sh TRANSMIT QUEUE STATISTICS
+For each transmit queue of each interface the following read-only statistics
+are provided:
+.Bl -tag -width "xxxxxx"
+.It Va dev.vtnet. Ns Ar X Ns Va .txq Ns Ar Y Ns Va .rescheduled
+The number of times the transmit interrupt handler was rescheduled.
+.It Va dev.vtnet. Ns Ar X Ns Va .txq Ns Ar Y Ns Va .tso
+The number of times TCP segment offloading was performed.
+.It Va dev.vtnet. Ns Ar X Ns Va .txq Ns Ar Y Ns Va .csum
+The number of times transmit checksum offloading for UDP or TCP was
+performed.
+.It Va dev.vtnet. Ns Ar X Ns Va .txq Ns Ar Y Ns Va .omcasts
+The number of multicast packets that were transmitted.
+.It Va dev.vtnet. Ns Ar X Ns Va .txq Ns Ar Y Ns Va .obytes
+The number of bytes that were transmitted (based on Ethernet frames).
+.It Va dev.vtnet. Ns Ar X Ns Va .txq Ns Ar Y Ns Va .opackets
+The number of packets that were transmitted (Ethernet frames).
+.El
+.Sh RECEIVE QUEUE STATISTICS
+For each receive queue of each interface the following read-only statistics
+are provided:
+.Bl -tag -width "xxxxxx"
+.It Va dev.vtnet. Ns Ar X Ns Va .rxq Ns Ar Y Ns Va .rescheduled
+The number of times the receive interrupt handler was rescheduled.
+.It Va dev.vtnet. Ns Ar X Ns Va .rxq Ns Ar Y Ns Va .host_lro
+The number of times TCP large receive offload was performed.
+.It Va dev.vtnet. Ns Ar X Ns Va .rxq Ns Ar Y Ns Va .csum_failed
+The number of times a packet with a request for receive or transmit checksum
+offloading was received and this request failed.
+The different reasons for the failure are counted by
+.Va dev.vtnet. Ns Ar X Ns Va .rx_csum_inaccessible_ipproto ,
+.Va dev.vtnet. Ns Ar X Ns Va .rx_csum_bad_ipproto ,
+.Va dev.vtnet. Ns Ar X Ns Va .rx_csum_bad_ethtype ,
+and
+.Va dev.vtnet. Ns Ar X Ns Va .rx_csum_bad_offset .
+.It Va dev.vtnet. Ns Ar X Ns Va .rxq Ns Ar Y Ns Va .csum
+The number of times receive checksum offloading for UDP or TCP was performed.
+.It Va dev.vtnet. Ns Ar X Ns Va .rxq Ns Ar Y Ns Va .ierrors
+The number of times an error occurred during input processing.
+.It Va dev.vtnet. Ns Ar X Ns Va .rxq Ns Ar Y Ns Va .iqdrops
+The number of times a packet was dropped during input processing.
+.It Va dev.vtnet. Ns Ar X Ns Va .rxq Ns Ar Y Ns Va .ibytes
+The number of bytes that were received (based on Ethernet frames).
+.It Va dev.vtnet. Ns Ar X Ns Va .rxq Ns Ar Y Ns Va .ipackets
+The number of packets that were received (Ethernet frames).
+.El
+.Sh INTERFACE TRANSMIT STATISTICS
+For each interface the following read-only transmit statistics are provided:
+.Bl -tag -width "xxxxxx"
+.It Va dev.vtnet. Ns Ar X Ns Va .tx_task_rescheduled
+The sum of
+.Va dev.vtnet. Ns Ar X Ns Va .txq Ns Ar Y Ns Va .rescheduled
+over all transmit queues of the interface.
+.It Va dev.vtnet. Ns Ar X Ns Va .tx_tso_offloaded
+The sum of
+.Va dev.vtnet. Ns Ar X Ns Va .txq Ns Ar Y Ns Va .tso
+over all transmit queues of the interface.
+.It Va dev.vtnet. Ns Ar X Ns Va .tx_csum_offloaded
+The sum of
+.Va dev.vtnet. Ns Ar X Ns Va .txq Ns Ar Y Ns Va .csum
+over all transmit queues of the interface.
+.It Va dev.vtnet. Ns Ar X Ns Va .tx_defrag_failed
+The number of times an attempt to defragment an mbuf chain failed during a
+transmit operation.
+.It Va dev.vtnet. Ns Ar X Ns Va .tx_defragged
+The number of times an mbuf chain was defragmented during a transmit operation.
+.It Va dev.vtnet. Ns Ar X Ns Va .tx_tso_without_csum
+The number of times TCP segment offloading was attempted without transmit
+checksum offloading.
+.It Va dev.vtnet. Ns Ar X Ns Va .tx_tso_not_tcp
+The number of times TCP segment offloading was attempted for a non-TCP packet.
+.It Va dev.vtnet. Ns Ar X Ns Va .tx_csum_proto_mismatch
+The number of times the IP protocol version of the transmit checksum
+offloading request did not match the IP protocol version of the packet.
+.It Va dev.vtnet. Ns Ar X Ns Va .tx_csum_unknown_ethtype
+The number of times a transmit offload operation was requested for an
+ethernet frame for which the EtherType was neither IPv4 nor IPv6
+(considering simple VLAN tagging).
+.El
+.Sh INTERFACE RECEIVE STATISTICS
+For each interface the following read-only receive statistics are provided:
+.Bl -tag -width "xxxxxx"
+.It Va dev.vtnet. Ns Ar X Ns Va .rx_task_rescheduled
+The sum of
+.Va dev.vtnet. Ns Ar X Ns Va .rxq Ns Ar Y Ns Va .rescheduled
+over all receive queues of the interface.
+.It Va dev.vtnet. Ns Ar X Ns Va .rx_csum_offloaded
+The sum of
+.Va dev.vtnet. Ns Ar X Ns Va .rxq Ns Ar Y Ns Va .csum
+over all receive queues of the interface.
+.It Va dev.vtnet. Ns Ar X Ns Va .rx_csum_failed
+The sum of
+.Va dev.vtnet. Ns Ar X Ns Va .rxq Ns Ar Y Ns Va .csum_failed
+over all receive queues of the interface.
+.It Va dev.vtnet. Ns Ar X Ns Va .rx_csum_inaccessible_ipproto
+The number of times a packet with a request for receive or transmit checksum
+offloading was received where the IP protocol was not accessible.
+.It Va dev.vtnet. Ns Ar X Ns Va .rx_csum_bad_offset
+The number of times fixing the checksum required by
+.Va hw.vtnet.fixup_needs_csum
+or
+.Va hw.vtnet. Ns Ar X Ns Va .fixup_needs_csum
+was attempted for a packet where the csum is not located in the first mbuf.
+.It Va dev.vtnet. Ns Ar X Ns Va .rx_csum_bad_ipproto
+The number of times a packet with a request for receive or transmit checksum
+offloading was received where the IP protocol was neither TCP nor UDP.
+.It Va dev.vtnet. Ns Ar X Ns Va .rx_csum_bad_ethtype
+The number of times a packet with a request for receive or transmit checksum
+offloading was received where the EtherType was neither IPv4 nor IPv6.
+.It Va dev.vtnet. Ns Ar X Ns Va .rx_mergeable_failed
+The number of times receiving a mergable buffer failed.
+.It Va dev.vtnet. Ns Ar X Ns Va .rx_enq_replacement_failed
+The number of times the enqueuing the replacement receive mbuf chain failed.
+.It Va dev.vtnet. Ns Ar X Ns Va .rx_frame_too_large
+The number of times the frame was loger than the mbuf chain during large
+receive offload without mergeable buffers.
+.It Va dev.vtnet. Ns Ar X Ns Va .mbuf_alloc_failed
+The number of times an mbuf cluster allocation for the receive buffer failed.
+.El
+.Sh INTERFACE CONFIGURATION PARAMETER
+For each interface the following read-only configuration parameters are
+provided:
+.Bl -tag -width "xxxxxx"
+.It Va dev.vtnet. Ns Ar X Ns Va .act_vq_pairs
+The number of active virtqueue pairs.
+.It Va dev.vtnet. Ns Ar X Ns Va .req_vq_pairs
+The number of requested virtqueue pairs.
+.It Va dev.vtnet. Ns Ar X Ns Va .max_vq_pairs
+The maximum number of supported virtqueue pairs.
+.It Va dev.vtnet. Ns Ar X Ns Va .flags
+The flags of the interface.
+Mostly for debugging purposes.
+.It Va dev.vtnet. Ns Ar X Ns Va .features
+The features of the interface as defined by the virtio specification.
+.El
+.Sh SEE ALSO
+.Xr arp 4 ,
+.Xr netintro 4 ,
+.Xr ng_ether 4 ,
+.Xr virtio 4 ,
+.Xr vlan 4 ,
+.Xr ifconfig 8
+.Sh HISTORY
+The
+.Nm
+driver was written by
+.An Bryan Venteicher Aq Mt bryanv@FreeBSD.org .
+It first appeared in
+.Fx 9.0 .
+.Sh CAVEATS
+The
+.Nm
+driver only supports LRO when the hypervisor advertises the
+mergeable buffer feature.