[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 5:35 PM John Fastabend <john.fastabend@gmail.com> wrote:
>
> Stanislav Fomichev wrote:
> > 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.
>
> I can't think of a use for it though so its just extra cycles. There
> is a helper to read the ktime.

As I mentioned in another reply, I wanted something SW-only to test
this whole metadata story.
The idea was:
- veth rx sets skb->tstamp (otherwise it's 0 at this point)
- veth kfunc to access rx_timestamp returns skb->tstamp
- xsk bpf program verifies that the metadata is non-zero
- the above shows end-to-end functionality with a software driver

> > Any way I can make it configurable? Is there some ethtool "enable tx
> > timestamping" option I can reuse?
>
> There is a -T option for timestamping in ethtool. There are also the
> socket controls for it. So you could spin up a socket and use it.
> But that is a bit broken as well I think it would be better if the
> timestamp came from the receiving physical nic?
>
> I have some mlx nics here and a k8s cluster with lots of veth
> devices so I could think a bit more. I'm just not sure why I would
> want the veth to timestamp things off hand?

-T is for dumping only it seems?

I'm probably using skb->tstamp in an unconventional manner here :-/
Do you know if enabling timestamping on the socket, as you suggest,
will get me some non-zero skb_hwtstamps with xsk?
I need something to show how the kfunc can return this data and how
can this data land in xdp prog / af_xdp chunk..


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