[if_vtnet] Add VIRTIO_NET_F_MTU flag support.

[afedorov]

Summary

This feature adds support for MTU host advice.

See details in https://reviews.freebsd.org/D23971

Test Plan

I tested this patch using bhyve + D23971

vmrun.sh -c 4 -m 4G -t tap1001,mtu=2000 -t tap1002,mtu=3000 -d fbsd-1.img freebsd-afedorov-1

root@fbsd-1:~ # ifconfig 
vtnet0: flags=8963<UP,BROADCAST,RUNNING,PROMISC,SIMPLEX,MULTICAST> metric 0 mtu 2000
        options=80028<VLAN_MTU,JUMBO_MTU,LINKSTATE>
        ether 00:a0:98:9e:d6:42
        inet 10.78.67.248 netmask 0xffffff00 broadcast 10.78.67.255
        media: Ethernet 10Gbase-T <full-duplex>
        status: active
        nd6 options=29<PERFORMNUD,IFDISABLED,AUTO_LINKLOCAL>
vtnet1: flags=8922<BROADCAST,PROMISC,SIMPLEX,MULTICAST> metric 0 mtu 3000
        options=80028<VLAN_MTU,JUMBO_MTU,LINKSTATE>
        ether 00:a0:98:de:5e:93
        media: Ethernet 10Gbase-T <full-duplex>
        status: active
        nd6 options=29<PERFORMNUD,IFDISABLED,AUTO_LINKLOCAL>
lo0: flags=8049<UP,LOOPBACK,RUNNING,MULTICAST> metric 0 mtu 16384
        options=680003<RXCSUM,TXCSUM,LINKSTATE,RXCSUM_IPV6,TXCSUM_IPV6>
        inet6 ::1 prefixlen 128
        inet6 fe80::1%lo0 prefixlen 64 scopeid 0x3
        inet 127.0.0.1 netmask 0xff000000
        groups: lo
        nd6 options=21<PERFORMNUD,AUTO_LINKLOCAL>

[bryanv]

I have added support for this feature (and more) in my VirtIO V1 branch that I plan on committing soon.

[afedorov]

Oh, I'm glad to hear that. May you comment on support from Bhyve side?

[jhb]

I'll defer to bryanv@ on whether this should be committed now or wait for the V1 patch.

sys/dev/virtio/network/if_vtnet.c

986

Nit: s/intial/initial/

[afedorov]

This is done in FreeBSD 13.