Is possible to encapsulate a new header in AF_XDP

[ZENOTME]

I'm trying to capture the packet from veth and encapsulate a new eth header to send it out by a real eth nic. As far as I know, in xdp ebpf, we can use bpf_xdp_adjust_head to do something like this. Is possible we can do it in userspace using AF_XDP? E.g. using user head room and adjust the addr of frame desc forward when we want to encapsulate a new header.

[ncshy]

It should be possible. The frame comes with a headroom by default (I can't be sure if it is 128B or 256B) so you can reference this by sending addr - (space for new headers) to process_packet() and fill in the new header there.
Another option is to push forward, so you use the same pkt pointer you get back from the example code as the start of your packet and push all the other data forward. Of course this will require a packet copy to hold the original data, which will reduce performance of your application.
Ultimately you cannot exceed your frame size and you have to keep track of you packet start offset, so that you fill in the tx descriptor correctly.