[xdp-hints] Re: [PATCH bpf-next v4 08/15] veth: Support RX XDP metadata

[Stanislav Fomichev <sdf@google.com>]

On Wed, Dec 14, 2022 at 10:09 AM Martin KaFai Lau <martin.lau@linux.dev> wrote:
>
> On 12/14/22 2:47 AM, Toke Høiland-Jørgensen wrote:
> > Jesper Dangaard Brouer <jbrouer@redhat.com> writes:
> >
> >> On 13/12/2022 21.42, Stanislav Fomichev wrote:
> >>> On Tue, Dec 13, 2022 at 7:55 AM Jesper Dangaard Brouer
> >>> <jbrouer@redhat.com> wrote:
> >>>>
> >>>>
> >>>> On 13/12/2022 03.35, Stanislav Fomichev wrote:
> >>>>> The goal is to enable end-to-end testing of the metadata for AF_XDP.
> >>>>>
> >>>>> Cc: John Fastabend <john.fastabend@gmail.com>
> >>>>> Cc: David Ahern <dsahern@gmail.com>
> >>>>> Cc: Martin KaFai Lau <martin.lau@linux.dev>
> >>>>> Cc: Jakub Kicinski <kuba@kernel.org>
> >>>>> Cc: Willem de Bruijn <willemb@google.com>
> >>>>> Cc: Jesper Dangaard Brouer <brouer@redhat.com>
> >>>>> Cc: Anatoly Burakov <anatoly.burakov@intel.com>
> >>>>> Cc: Alexander Lobakin <alexandr.lobakin@intel.com>
> >>>>> Cc: Magnus Karlsson <magnus.karlsson@gmail.com>
> >>>>> Cc: Maryam Tahhan <mtahhan@redhat.com>
> >>>>> Cc: xdp-hints@xdp-project.net
> >>>>> Cc: netdev@vger.kernel.org
> >>>>> Signed-off-by: Stanislav Fomichev <sdf@google.com>
> >>>>> ---
> >>>>>     drivers/net/veth.c | 24 ++++++++++++++++++++++++
> >>>>>     1 file changed, 24 insertions(+)
> >>>>>
> >>>>> diff --git a/drivers/net/veth.c b/drivers/net/veth.c
> >>>>> index 04ffd8cb2945..d5491e7a2798 100644
> >>>>> --- a/drivers/net/veth.c
> >>>>> +++ b/drivers/net/veth.c
> >>>>> @@ -118,6 +118,7 @@ static struct {
> >>>>>
> >>>>>     struct veth_xdp_buff {
> >>>>>         struct xdp_buff xdp;
> >>>>> +     struct sk_buff *skb;
> >>>>>     };
> >>>>>
> >>>>>     static int veth_get_link_ksettings(struct net_device *dev,
> >>>>> @@ -602,6 +603,7 @@ static struct xdp_frame *veth_xdp_rcv_one(struct veth_rq *rq,
> >>>>>
> >>>>>                 xdp_convert_frame_to_buff(frame, xdp);
> >>>>>                 xdp->rxq = &rq->xdp_rxq;
> >>>>> +             vxbuf.skb = NULL;
> >>>>>
> >>>>>                 act = bpf_prog_run_xdp(xdp_prog, xdp);
> >>>>>
> >>>>> @@ -823,6 +825,7 @@ static struct sk_buff *veth_xdp_rcv_skb(struct veth_rq *rq,
> >>>>>         __skb_push(skb, skb->data - skb_mac_header(skb));
> >>>>>         if (veth_convert_skb_to_xdp_buff(rq, xdp, &skb))
> >>>>>                 goto drop;
> >>>>> +     vxbuf.skb = skb;
> >>>>>
> >>>>>         orig_data = xdp->data;
> >>>>>         orig_data_end = xdp->data_end;
> >>>>> @@ -1601,6 +1604,21 @@ static int veth_xdp(struct net_device *dev, struct netdev_bpf *xdp)
> >>>>>         }
> >>>>>     }
> >>>>>
> >>>>> +static int veth_xdp_rx_timestamp(const struct xdp_md *ctx, u64 *timestamp)
> >>>>> +{
> >>>>> +     *timestamp = ktime_get_mono_fast_ns();
> >>>>
> >>>> This should be reading the hardware timestamp in the SKB.
> >>>>
> >>>> Details: This hardware timestamp in the SKB is located in
> >>>> skb_shared_info area, which is also available for xdp_frame (currently
> >>>> used for multi-buffer purposes).  Thus, when adding xdp-hints "store"
> >>>> functionality, it would be natural to store the HW TS in the same place.
> >>>> Making the veth skb/xdp_frame code paths able to share code.
> >>>
> >>> Does something like the following look acceptable as well?
> >>>
> >>> *timestamp = skb_hwtstamps(_ctx->skb)->hwtstamp;
>
> If it is to test the kfunc and ensure veth_xdp_rx_timestamp is called, this
> alone should be enough. skb_hwtstamps(_ctx->skb)->hwtstamp should be 0 if
> hwtstamp is unavailable?  The test can initialize the 'u64 *timestamp' arg to
> non-zero first.  If it is not good enough, an fentry tracing can be done to
> veth_xdp_rx_timestamp to ensure it is called also.  There is also fmod_ret that
> could change the return value but the timestamp is not the return value though.
>
> If the above is not enough, another direction of thought could be doing it
> through bpf_prog_test_run_xdp() but it will need a way to initialize the
> veth_xdp_buff.
>
> That said, overall, I don't think it is a good idea to bend the
> veth_xdp_rx_timestamp kfunc too much only for testing purpose unless there is no
> other way out.

Oh, good point about just making sure veth_xdp_rx_timestamp returns
timestamp=0. That should be enough, thanks!