On 07/02, Song, Yoong Siang wrote:
On Wednesday, July 2, 2025 11:19 PM, Stanislav Fomichev stfomichev@gmail.com wrote:
On 07/02, Song, Yoong Siang wrote:
On Wednesday, July 2, 2025 10:23 AM, Song, Yoong Siang
yoong.siang.song@intel.com wrote:
On Wednesday, July 2, 2025 12:31 AM, Stanislav Fomichev
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@intel.com
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@intel.com/
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!