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

["Toke Høiland-Jørgensen" <toke@redhat.com>]

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 (!*timestamp)
>>          *timestamp = ktime_get_mono_fast_ns(); /* sw fallback */
>> 
>
> How can the BPF programmer tell the difference between getting hardware 
> or software timestamp?
>
> This will get really confusing when someone implements a tcpdump feature
> (like/extend xdpdump) and some packets (e.g. PTP packets) have HW
> timestamps and some don't.  The time sequence in the pcap will be strange.
>
>> Because I'd like to be able to test this path in the selftests. As
>> long as I get some number from veth_xdp_rx_timestamp, I can test it.
>> No amount of SOF_TIMESTAMPING_{SOFTWARE,RX_SOFTWARE,RAW_HARDWARE}
>> triggers non-zero hwtstamp for xsk receive path. Any suggestions?
>> 
>
> You could implement the "store" operation I mentioned before.
> For testing you can store an arbitrary value in the timestamp and check
> it later by reading it back.
>
> I can see you have changed the API to send down a pointer. Thus, a
> simple flag could implement the writing the provided timestamp.
>
> Regarding flags for reading the timestamp.  Should we be able to specify
> what clock type we are asking for?
> Have you notice that tcpdump can ask for different types of
> timestamps[1]. e.g. for hardware timestamps it is either
> 'adapter_unsynced' or 'adaptor'. (See semantic in [1])
>
>   # tcpdump -ni eth1 -j adapter_unsynced --time-stamp-precision=nano

I don't think it makes sense for XDP to *ask* for a specific timestamp
(any individual packet probably only has one, right?). But we could add
a way to query which type of timestamp is available, either as a second
argument to the same function, or as a separate one. Same thing for the
hash, BTW (skb_set_hash() also takes a type argument).

I guess the easiest would be to just add a second parameter to both
getter functions for the type, but maybe there's a performance argument
for having it be a separate kfunc (at least for timestamp)?

-Toke