On 21/09/2024 22.17, Alexander Lobakin wrote:
From: Lorenzo Bianconi <lorenzo@xxxxxxxxxx>
Date: Sat, 21 Sep 2024 18:52:56 +0200
This series introduces the xdp_rx_meta struct in the xdp_buff/xdp_frame
&xdp_buff is on the stack.
&xdp_frame consumes headroom.
IOW they're size-sensitive and putting metadata directly there might
play bad; if not now, then later.
Our idea (me + Toke) was as follows:
- new BPF kfunc to build generic meta. If called, the driver builds a
generic meta with hash, csum etc., in the data_meta area.
I do agree that it should be the XDP prog (via a new BPF kfunc) that
decide if xdp_frame should be updated to contain a generic meta struct.
*BUT* I think we should use the xdp_frame area, and not the
xdp->data_meta area.
A details is that I think this kfunc should write data directly into
xdp_frame area, even then we are only operating on the xdp_buff, as we
do have access to the area xdp_frame will be created in.
When using data_meta area, then netstack encap/decap needs to move the
data_meta area (extra cycles). The xdp_frame area (live in top) don't
have this issue.
It is easier to allow xdp_frame area to survive longer together with the
SKB. Today we "release" this xdp_frame area to be used by SKB for extra
headroom (see xdp_scrub_frame). I can imagine that we can move SKB
fields to this area, and reduce the size of the SKB alloc. (This then
becomes the mini-SKB we discussed a couple of years ago).
Yes, this also consumes headroom, but only when the corresponding func
is called. Introducing new fields like you're doing will consume it
unconditionally;
We agree on the kfunc call marks area as consumed/in-use. We can extend
xdp_frame statically like Lorenzo does (with struct xdp_rx_meta), but
xdp_frame->flags can be used for marking this area as used or not.
- when &xdp_frame gets converted to sk_buff, the function checks whether
data_meta contains a generic structure filled with hints.
Agree, but take data from xdp_frame->xdp_rx_meta.
When XDP returns XDP_PASS, then I also want to see this data applied to
the SKB. In patchset[1] Yan called this xdp_frame_fixup_skb_offloading()
and xdp_buff_fixup_skb_offloading(). (Perhaps "fixup" isn't the right
term, "apply" is perhaps better). Having this generic-name allow us to
extend with newer offloads, and eventually move members out of SKB.
We called it "fixup", because our use-case is that our XDP load-balancer
(Unimog) XDP_TX bounce packets with in GRE header encap, and on the
receiving NIC (due to encap) we lost the HW hash/csum, which we want to
transfer from the original NIC, decap in XDP and apply the original HW
hash/csum via this "fixup" call.
--Jesper
[1] https://lore.kernel.org/all/cover.1718919473.git.yan@xxxxxxxxxxxxxx/
We also thought about &skb_shared_info, but it's also size-sensitive as
it consumes tailroom.
one as a container to store the already supported xdp rx hw hints (rx_hash
and rx_vlan, rx_timestamp will be stored in skb_shared_info area) when the
eBPF program running on the nic performs XDP_REDIRECT. Doing so, we are able
to set the skb metadata converting the xdp_buff/xdp_frame to a skb.
Thanks,
Olek
