bhyve: add checksum offloading for tap

[timo.voelker_fh-muenster.de]

Summary

To allow the use of checksum offloading for packets to and from a VM, this patch changes the following.

• Set the vnet_hdr_len > 0 for tap.
• Report that the tap backend supports receiving packets with partial checksum (VIRTIO_NET_F_CSUM) and that the guest can get packets with partial checksum (VIRTIO_NET_F_GUEST_CSUM).
• Enable transmission checksum offloading (IFCAP_TXCSUM and IFCAP_TXCSUM) on the tap interface if the feature VIRTIO_NET_F_GUEST_CSUM has been negotiated.

Note that if the VM sends a packet with partial checksum and the host sends it out over an interface that does not support checksum offloading, the packet go out with an incorrect checksum. D56564 moves the check of whether an interface supports checksum offloading close to the driver and prevents this issue.

Test Plan

Create a tap interface.

ifconfig tap0 create

Start a FreeBSD VM with bhyve using this tap interface. The tap0 interface on the host and the vtnet0 interface in the VM should have txcsum and txcsum6 enabled. Configure an IP address on both interfaces.

Host

ifconfig tap0 inet 192.168.0.1/24

VM

ifconfig vtnet0 inet 192.168.0.2/24

Use nc for a test with TCP packets. In the VM run

nc -l 1234

On the host, run

nc 192.168.0.2 1234

Result: Both the host and the VM offload the computation of the TCP checksum. Host and VM can communicate without computing the TCP checksum.

[markj]

usr.sbin/bhyve/net_backends.c

93

This value causes the assertion on lines 429-430 to fail if the frontend negotiates mergeable rx buffers with the guest virtio driver.

[timo.voelker_fh-muenster.de]

Update the vnet_hdr_len in tap_set_cap().

Change the way of enabling transmission checksum offloading (see new summary).

[timo.voelker_fh-muenster.de]

usr.sbin/bhyve/net_backends.c

93

Thanks, that helped. Now, I update the vnet_hdr_len in tap_set_cap().