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

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Date:   Wed, 09 Nov 2022 16:25:56 -0800
From:   John Fastabend <john.fastabend@...il.com>
To:     Stanislav Fomichev <sdf@...gle.com>, bpf@...r.kernel.org
Cc:     ast@...nel.org, daniel@...earbox.net, andrii@...nel.org,
        martin.lau@...ux.dev, song@...nel.org, yhs@...com,
        john.fastabend@...il.com, kpsingh@...nel.org, sdf@...gle.com,
        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:
> 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.

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