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

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Date:   Wed, 09 Nov 2022 17:35:21 -0800
From:   John Fastabend <john.fastabend@...il.com>
To:     Stanislav Fomichev <sdf@...gle.com>,
        John Fastabend <john.fastabend@...il.com>
Cc:     bpf@...r.kernel.org, ast@...nel.org, daniel@...earbox.net,
        andrii@...nel.org, martin.lau@...ux.dev, song@...nel.org,
        yhs@...com, kpsingh@...nel.org, haoluo@...gle.com,
        jolsa@...nel.org, David Ahern <dsahern@...il.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Willem de Bruijn <willemb@...gle.com>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        Anatoly Burakov <anatoly.burakov@...el.com>,
        Alexander Lobakin <alexandr.lobakin@...el.com>,
        Magnus Karlsson <magnus.karlsson@...il.com>,
        Maryam Tahhan <mtahhan@...hat.com>, xdp-hints@...-project.net,
        netdev@...r.kernel.org
Subject: Re: [RFC bpf-next v2 04/14] veth: Support rx timestamp metadata for
 xdp

Stanislav Fomichev wrote:
> On Wed, Nov 9, 2022 at 4:26 PM John Fastabend <john.fastabend@...il.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.

> 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?

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