On Thursday, July 3, 2025 12:04 AM, Stanislav Fomichev <stfomichev@xxxxxxxxx> wrote:
>On 07/02, Song, Yoong Siang wrote:
>> On Wednesday, July 2, 2025 11:19 PM, Stanislav Fomichev
><stfomichev@xxxxxxxxx> wrote:
>> >On 07/02, Song, Yoong Siang wrote:
>> >> On Wednesday, July 2, 2025 10:23 AM, Song, Yoong Siang
>> ><yoong.siang.song@xxxxxxxxx> wrote:
>> >> >On Wednesday, July 2, 2025 12:31 AM, Stanislav Fomichev
>> ><stfomichev@xxxxxxxxx>
>> >> >wrote:
>> >> >>On 07/01, Song Yoong Siang wrote:
>> >> >>> Introduce the XDP_METADATA_SIZE macro to ensure that user applications
>can
>> >> >>> consistently retrieve the correct location of struct xdp_meta.
>> >> >>>
>> >> >>> Prior to this commit, the XDP program adjusted the data_meta backward by
>> >> >>> the size of struct xdp_meta, while the user application retrieved the data
>> >> >>> by calculating backward from the data pointer. This approach only worked if
>> >> >>> xdp_buff->data_meta was equal to xdp_buff->data before calling
>> >> >>> bpf_xdp_adjust_meta.
>> >> >>>
>> >> >>> With the introduction of XDP_METADATA_SIZE, both the XDP program and
>user
>> >> >>> application now calculate and identify the location of struct xdp_meta from
>> >> >>> the data pointer. This ensures the implementation remains functional even
>> >> >>> when there is device-reserved metadata, making the tests more portable
>> >> >>> across different NICs.
>> >> >>>
>> >> >>> Signed-off-by: Song Yoong Siang <yoong.siang.song@xxxxxxxxx>
>> >> >>> ---
>> >> >>> tools/testing/selftests/bpf/prog_tests/xdp_metadata.c | 2 +-
>> >> >>> tools/testing/selftests/bpf/progs/xdp_hw_metadata.c | 10 +++++++++-
>> >> >>> tools/testing/selftests/bpf/progs/xdp_metadata.c | 8 +++++++-
>> >> >>> tools/testing/selftests/bpf/xdp_hw_metadata.c | 2 +-
>> >> >>> tools/testing/selftests/bpf/xdp_metadata.h | 7 +++++++
>> >> >>> 5 files changed, 25 insertions(+), 4 deletions(-)
>> >> >>>
>> >> >>> diff --git a/tools/testing/selftests/bpf/prog_tests/xdp_metadata.c
>> >> >>b/tools/testing/selftests/bpf/prog_tests/xdp_metadata.c
>> >> >>> index 19f92affc2da..8d6c2633698b 100644
>> >> >>> --- a/tools/testing/selftests/bpf/prog_tests/xdp_metadata.c
>> >> >>> +++ b/tools/testing/selftests/bpf/prog_tests/xdp_metadata.c
>> >> >>> @@ -302,7 +302,7 @@ static int verify_xsk_metadata(struct xsk *xsk, bool
>> >> >>sent_from_af_xdp)
>> >> >>>
>> >> >>> /* custom metadata */
>> >> >>>
>> >> >>> - meta = data - sizeof(struct xdp_meta);
>> >> >>> + meta = data - XDP_METADATA_SIZE;
>> >> >>>
>> >> >>> if (!ASSERT_NEQ(meta->rx_timestamp, 0, "rx_timestamp"))
>> >> >>> return -1;
>> >> >>> diff --git a/tools/testing/selftests/bpf/progs/xdp_hw_metadata.c
>> >> >>b/tools/testing/selftests/bpf/progs/xdp_hw_metadata.c
>> >> >>> index 330ece2eabdb..72242ac1cdcd 100644
>> >> >>> --- a/tools/testing/selftests/bpf/progs/xdp_hw_metadata.c
>> >> >>> +++ b/tools/testing/selftests/bpf/progs/xdp_hw_metadata.c
>> >> >>> @@ -27,6 +27,7 @@ extern int bpf_xdp_metadata_rx_vlan_tag(const
>struct
>> >> >>xdp_md *ctx,
>> >> >>> SEC("xdp.frags")
>> >> >>> int rx(struct xdp_md *ctx)
>> >> >>> {
>> >> >>> + int metalen_used, metalen_to_adjust;
>> >> >>> void *data, *data_meta, *data_end;
>> >> >>> struct ipv6hdr *ip6h = NULL;
>> >> >>> struct udphdr *udp = NULL;
>> >> >>> @@ -72,7 +73,14 @@ int rx(struct xdp_md *ctx)
>> >> >>> return XDP_PASS;
>> >> >>> }
>> >> >>>
>> >> >>> - err = bpf_xdp_adjust_meta(ctx, -(int)sizeof(struct xdp_meta));
>> >> >>
>> >> >>[..]
>> >> >>
>> >> >>> + metalen_used = ctx->data - ctx->data_meta;
>> >> >>
>> >> >>Is the intent here to query how much metadata has been consumed/reserved
>> >> >>by the driver?
>> >> >Yes.
>> >> >
>> >> >>Looking at IGC it has the following code/comment:
>> >> >>
>> >> >> bi->xdp->data += IGC_TS_HDR_LEN;
>> >> >>
>> >> >> /* HW timestamp has been copied into local variable. Metadata
>> >> >> * length when XDP program is called should be 0.
>> >> >> */
>> >> >> bi->xdp->data_meta += IGC_TS_HDR_LEN;
>> >> >>
>> >> >>Are you sure that metadata size is correctly exposed to the bpf program?
>> >> >You are right, the current igc driver didn't expose the metadata size correctly.
>> >> >I submitted [1] to fix it.
>> >> >
>> >> >[1] https://patchwork.ozlabs.org/project/intel-wired-
>> >> >lan/patch/20250701080955.3273137-1-yoong.siang.song@xxxxxxxxx/
>> >> >
>> >> >>
>> >> >>My assumptions was that we should just unconditionally do
>> >bpf_xdp_adjust_meta
>> >> >>with -XDP_METADATA_SIZE and that should be good enough.
>> >> >
>> >> >The checking is just for precautions. No problem if directly adjust the meta
>> >> >unconditionally.
>> >> >That will save processing time for each packet as well.
>> >> >I will remove the checking and submit v2.
>> >> >
>> >> >Thanks & Regards
>> >> >Siang
>> >> >
>> >>
>> >> Hi Stanislav Fomichev,
>> >>
>> >> I submitted v2. But after that, I think twice. IMHO,
>> >> err = bpf_xdp_adjust_meta(ctx, (int)(ctx->data - ctx->data_meta -
>> >XDP_METADATA_SIZE));
>> >> is better than
>> >> err = bpf_xdp_adjust_meta(ctx, -(int)XDP_METADATA_SIZE);
>> >> because it is more robust.
>> >>
>> >> Any thoughts?
>> >
>> >My preference is on keeping everything as is and converting to
>> >-(int)XDP_METADATA_SIZE. Making IGC properly expose (temporary) metadata
>len
>> >is a user visible change, not sure we have a good justification?
>>
>> Thank you for your feedback. I agree that we don't have a strong justification
>> for making the metadata length user-visible at this time. I concur with your
>> preference to keep everything as is and proceed with -(int)XDP_METADATA_SIZE.
>>
>> Btw, do you think whether my first patch which changes the documentation is
>> still needed or not?
>
>Yes, the documentation is super useful, let's keep it!
Sure. I will keep the documentation but submit v3 to remove the portion that
suggest user to use bpf_xdp_adjust_meta with METADATA_SIZE - (xdp_buff->data - xdp_buff->data_meta).
