Lorenzo Bianconi wrote:

[...]

I just read the commit messages for v8 so far. But, I'm still wondering how
to handle use cases where we want to put extra bytes at the end of the
packet, or really anywhere in the general case. We can extend tail with above
is there anyway to then write into that extra space?

I think most use cases will only want headers so we can likely make it 
a callout to a helper. Could we add something like, xdp_get_bytes(start, end)
to pull in the bytes?

My dumb pseudoprogram being something like,

  trailer[16] = {0,1,2,3,4,5,6,7,8,9,a,b,c,d,e}
  trailer_size = 16;
  old_end = xdp->length;
  new_end = xdp->length + trailer_size;

  err = bpf_xdp_adjust_tail(xdp, trailer_size)
  if (err) return err;

  err = xdp_get_bytes(xdp, old_end, new_end);
  if (err) return err;

  memcpy(xdp->data, trailer, trailer_size);

Do you think that could work if we code up xdp_get_bytes()? Does the driver
have enough context to adjust xdp to map to my get_bytes() call? I think
so but we should check.

Hi John,

can you please give more details about how xdp_get_bytes() is expected to work?
iiuc trailer will be pulled at the beginning of the frame after updating the
xdp_buff with xdp_get_bytes helper, correct?
If so I guess it will doable, it is just a matter of reserve more space mapping
the buffer on the dma engine respect to XDP_PACKET_HEADROOM. If the frame does
not fit in a single buffer, it will be split over multiple buffers.
If you are referring to add trailer at the end of the buffer, I guess it is
doable as well introducing a bpf helper.
I guess both of the solutions are orthogonal to this series.

Regards,
Lorenzo

quoted

More info about the main idea behind this approach can be found here [1][2].

Thanks for working on this!