[xdp-hints] Re: [RFC bpf-next v2 04/14] veth: Support rx timestamp metadata for xdp

[Stanislav Fomichev <sdf@google.com>]

On Wed, Nov 9, 2022 at 4:26 PM John Fastabend <john.fastabend@gmail.com> wrote:
>
> Stanislav Fomichev wrote:
> > xskxceiver conveniently setups up veth pairs so it seems logical
> > to use veth as an example for some of the metadata handling.
> >
> > We timestamp skb right when we "receive" it, store its
> > pointer in new veth_xdp_buff wrapper and generate BPF bytecode to
> > reach it from the BPF program.
> >
> > This largely follows the idea of "store some queue context in
> > the xdp_buff/xdp_frame so the metadata can be reached out
> > from the BPF program".
> >
>
> [...]
>
> >       orig_data = xdp->data;
> >       orig_data_end = xdp->data_end;
> > +     vxbuf.skb = skb;
> >
> >       act = bpf_prog_run_xdp(xdp_prog, xdp);
> >
> > @@ -942,6 +946,7 @@ static int veth_xdp_rcv(struct veth_rq *rq, int budget,
> >                       struct sk_buff *skb = ptr;
> >
> >                       stats->xdp_bytes += skb->len;
> > +                     __net_timestamp(skb);
>
> Just getting to reviewing in depth a bit more. But we hit veth with lots of
> packets in some configurations I don't think we want to add a __net_timestamp
> here when vast majority of use cases will have no need for timestamp on veth
> device. I didn't do a benchmark but its not free.
>
> If there is a real use case for timestamping on veth we could do it through
> a XDP program directly? Basically fallback for devices without hw timestamps.
> Anyways I need the helper to support hardware without time stamping.
>
> Not sure if this was just part of the RFC to explore BPF programs or not.

Initially I've done it mostly so I can have selftests on top of veth
driver, but I'd still prefer to keep it to have working tests.
Any way I can make it configurable? Is there some ethtool "enable tx
timestamping" option I can reuse?

> >                       skb = veth_xdp_rcv_skb(rq, skb, bq, stats);
> >                       if (skb) {
> >                               if (skb_shared(skb) || skb_unclone(skb, GFP_ATOMIC))