Re: [PATCH bpf-next V2 5/5] selftests/bpf: xdp_hw_metadata track more timestamps

Jesper Dangaard Brouer
2023-04-19T16:41:59Z

On 18/04/2023 18.36, Stanislav Fomichev wrote:
> On 04/18, Jesper Dangaard Brouer wrote:
>> To correlate the hardware RX timestamp with something, add tracking of
>> two software timestamps both clock source CLOCK_TAI (see description in
>> man clock_gettime(2)).
>>
>> XDP metadata is extended with xdp_timestamp for capturing when XDP
>> received the packet. Populated with BPF helper bpf_ktime_get_tai_ns(). I
>> could not find a BPF helper for getting CLOCK_REALTIME, which would have
>> been preferred. In userspace when AF_XDP sees the packet another
>> software timestamp is recorded via clock_gettime() also clock source
>> CLOCK_TAI.
>>
[...]

>> More explanation of the output and how this can be used to identify
>> clock drift for the HW clock can be seen here[1]:
>>
>> [1]https://github.com/xdp-project/xdp-project/blob/master/areas/hints/xdp_hints_kfuncs02_driver_igc.org
>>
>> Signed-off-by: Jesper Dangaard Brouer<brouer@redhat.com>
> Acked-by: Stanislav Fomichev<sdf@google.com>
>
>> ---
>> .../testing/selftests/bpf/progs/xdp_hw_metadata.c | 4 +-
>> tools/testing/selftests/bpf/xdp_hw_metadata.c | 47 ++++++++++++++++++--
>> tools/testing/selftests/bpf/xdp_metadata.h | 1
>> 3 files changed, 46 insertions(+), 6 deletions(-)
>>
>> diff --git a/tools/testing/selftests/bpf/progs/xdp_hw_metadata.c b/tools/testing/selftests/bpf/progs/xdp_hw_metadata.c
>> index e1c787815e44..b2dfd7066c6e 100644
>> --- a/tools/testing/selftests/bpf/progs/xdp_hw_metadata.c
>> +++ b/tools/testing/selftests/bpf/progs/xdp_hw_metadata.c
>> @@ -77,7 +77,9 @@ int rx(struct xdp_md *ctx)
>> }
>>
>> err = bpf_xdp_metadata_rx_timestamp(ctx, &meta->rx_timestamp);
>> - if (err)
> [..]
>
>> + if (!err)
>> + meta->xdp_timestamp = bpf_ktime_get_tai_ns();
> nit: why not set it unconditionally?

Because userspace application doesn't use it for anything, when
meta->rx_timestamp is zero.

--Jesper