Re: regression: UFO removal breaks kvm live migration

quoted

Okay, I will send a patch to reinstate UFO for this use case (only). There
is some related work in tap_handle_frame and packet_direct_xmit to
segment directly in the device. I will be traveling the next few days, so
it won't be in time for 4.14 (but can go in stable later, of course).

I'm finishing up and running some tests. The majority of the patch is a
straightforward partial revert of the patchset, so while fairly large for a
patch to net (~150 lines, esp. in udp[46]_ufo_fragment), that is all
thoroughly tested code. Notably absent are the protocol layer and
hardware support (NETIF_F_UFO) portions.

The only open issue is whether to rely on existing skb_gso_segment
processing in the transmit path from validate_xmit_skb or to add new
skb_gso_segment calls directly to tun_get_user, tap_get_user and
pf_packet. Tun has to loop around four different ways of injecting
packets into the device. Something like the below snippet.

More conservative is to introduce no completely new code and rely on
validate_xmit_skb, but that means having to protect the entire stack
against skbs with SKB_GSO_UDP, so also bringing back some
checksum and fragment handling snippets in gre_gso_segment,
__skb_udp_tunnel_segment, act_csum and openvswitch.

Come to think of it, as this patch does not bring back NETIF_F_UFO
support to NETIF_F_GSO_SOFTWARE, the tunnel cases can be
excluded.

Then this is probably the simpler and more obviously correct approach.