Re: [PATCH bpf-next 2/2] selftests/bpf: Enhance XDP Rx Metadata Handling

[Stanislav Fomichev]

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? 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?

My assumptions was that we should just unconditionally do bpf_xdp_adjust_meta
with -XDP_METADATA_SIZE and that should be good enough.