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Message-ID: <77463344-1b1a-443a-97be-a7ef8a88b8af@kernel.org>
Date: Thu, 3 Jul 2025 19:04:40 +0200
From: Jesper Dangaard Brouer <hawk@...nel.org>
To: Song Yoong Siang <yoong.siang.song@...el.com>,
"David S . Miller" <davem@...emloft.net>, Eric Dumazet
<edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>,
Paolo Abeni <pabeni@...hat.com>, Simon Horman <horms@...nel.org>,
Jonathan Corbet <corbet@....net>, Alexei Starovoitov <ast@...nel.org>,
Daniel Borkmann <daniel@...earbox.net>,
John Fastabend <john.fastabend@...il.com>,
Stanislav Fomichev <sdf@...ichev.me>, Andrii Nakryiko <andrii@...nel.org>,
Martin KaFai Lau <martin.lau@...ux.dev>, Eduard Zingerman
<eddyz87@...il.com>, Song Liu <song@...nel.org>,
Yonghong Song <yonghong.song@...ux.dev>, KP Singh <kpsingh@...nel.org>,
Hao Luo <haoluo@...gle.com>, Jiri Olsa <jolsa@...nel.org>,
Mykola Lysenko <mykolal@...com>, Shuah Khan <shuah@...nel.org>
Cc: netdev@...r.kernel.org, linux-doc@...r.kernel.org,
linux-kernel@...r.kernel.org, bpf@...r.kernel.org,
linux-kselftest@...r.kernel.org
Subject: Re: [PATCH bpf-next,v3 2/2] selftests/bpf: Enhance XDP Rx metadata
handling
On 02/07/2025 18.57, Song Yoong Siang wrote:
> Introduce the XDP_METADATA_SIZE macro as a conservative measure to
> accommodate any metadata areas reserved by Ethernet devices.
>
This seems like a sloppy workaround :-(
To me, the problem arise because AF_XDP is lacking the ability to
communicate the size of the data_meta area. If we had this capability,
then we could allow the IGC driver to take some of the space, have the
BPF-prog expand it futher (bpf_xdp_adjust_meta) and then userspace
AF_XDP would simply be able to see the size of the data_meta area, and
apply the struct xdp_meta at right offset.
> Signed-off-by: Song Yoong Siang <yoong.siang.song@...el.com>
> ---
> tools/testing/selftests/bpf/prog_tests/xdp_metadata.c | 2 +-
> tools/testing/selftests/bpf/progs/xdp_hw_metadata.c | 2 +-
> tools/testing/selftests/bpf/progs/xdp_metadata.c | 2 +-
> tools/testing/selftests/bpf/xdp_hw_metadata.c | 2 +-
> tools/testing/selftests/bpf/xdp_metadata.h | 7 +++++++
> 5 files changed, 11 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..3766f58d3486 100644
> --- a/tools/testing/selftests/bpf/progs/xdp_hw_metadata.c
> +++ b/tools/testing/selftests/bpf/progs/xdp_hw_metadata.c
> @@ -72,7 +72,7 @@ int rx(struct xdp_md *ctx)
> return XDP_PASS;
> }
>
> - err = bpf_xdp_adjust_meta(ctx, -(int)sizeof(struct xdp_meta));
> + err = bpf_xdp_adjust_meta(ctx, -(int)XDP_METADATA_SIZE);
> if (err) {
> __sync_add_and_fetch(&pkts_fail, 1);
> return XDP_PASS;
> diff --git a/tools/testing/selftests/bpf/progs/xdp_metadata.c b/tools/testing/selftests/bpf/progs/xdp_metadata.c
> index 09bb8a038d52..5cada85fe0f4 100644
> --- a/tools/testing/selftests/bpf/progs/xdp_metadata.c
> +++ b/tools/testing/selftests/bpf/progs/xdp_metadata.c
> @@ -73,7 +73,7 @@ int rx(struct xdp_md *ctx)
>
> /* Reserve enough for all custom metadata. */
>
> - ret = bpf_xdp_adjust_meta(ctx, -(int)sizeof(struct xdp_meta));
> + ret = bpf_xdp_adjust_meta(ctx, -(int)XDP_METADATA_SIZE);
> if (ret != 0)
> return XDP_DROP;
>
> diff --git a/tools/testing/selftests/bpf/xdp_hw_metadata.c b/tools/testing/selftests/bpf/xdp_hw_metadata.c
> index 3d8de0d4c96a..a529d55d4ff4 100644
> --- a/tools/testing/selftests/bpf/xdp_hw_metadata.c
> +++ b/tools/testing/selftests/bpf/xdp_hw_metadata.c
> @@ -223,7 +223,7 @@ static void verify_xdp_metadata(void *data, clockid_t clock_id)
> {
> struct xdp_meta *meta;
>
> - meta = data - sizeof(*meta);
> + meta = data - XDP_METADATA_SIZE;
>
> if (meta->hint_valid & XDP_META_FIELD_RSS)
> printf("rx_hash: 0x%X with RSS type:0x%X\n",
> diff --git a/tools/testing/selftests/bpf/xdp_metadata.h b/tools/testing/selftests/bpf/xdp_metadata.h
> index 87318ad1117a..2dfd3bf5e7bb 100644
> --- a/tools/testing/selftests/bpf/xdp_metadata.h
> +++ b/tools/testing/selftests/bpf/xdp_metadata.h
> @@ -50,3 +50,10 @@ struct xdp_meta {
> };
> enum xdp_meta_field hint_valid;
> };
> +
> +/* XDP_METADATA_SIZE must be at least the size of struct xdp_meta. An additional
> + * 32 bytes of padding is included as a conservative measure to accommodate any
> + * metadata areas reserved by Ethernet devices. If the device-reserved metadata
> + * exceeds 32 bytes, this value will need adjustment.
> + */
> +#define XDP_METADATA_SIZE (sizeof(struct xdp_meta) + 32)
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