The ethtool manpage only gives a nebulous explanation:
rxvlan on|off
Specifies whether RX VLAN acceleration should be enabled
txvlan on|off
Specifies whether TX VLAN acceleration should be enabled
What exactly do the options accomplish, assuming you can enable them?
Apparently, rxvlan and txvlan are aliases for the kernel features rx-vlan-hw-parse and tx-vlan-hw-insert respectively (see ethtool.c).
In the kernel they are translated to the netdev features NETIF_F_HW_VLAN_CTAG_RX_BIT and NETIF_F_HW_VLAN_CTAG_TX_BIT. (See net/core/ethtool.c)
To the best of my knowledge, the TX_BIT allows the NIC to add the VLAN tag during transmission without explicitly placing it in the packet. See the relevant code in the VLAN driver.
Similarly, when the RX_BIT is enabled, the NIC can strip the VLAN tag from incoming packets and let the driver place the information from the VLAN tag in the relevant skb.
