[PATCH bpf-next 0/2] Clarify and Enhance XDP Rx Metadata Handling

[PATCH bpf-next 0/2] Clarify and Enhance XDP Rx Metadata Handling

[Song Yoong Siang]

This patch set improves the documentation and selftests for XDP Rx metadata
handling. The first patch clarifies the documentation around XDP metadata
layout and the use of bpf_xdp_adjust_meta. The second patch enhances the
BPF selftests to make XDP metadata handling more robust and portable across
different NICs.

Prior to this patch set, the user application retrieved the xdp_meta by
calculating backward from the data pointer, while the XDP program fill in
the xdp_meta by calculating backward from data_meta. This approach will
cause mismatch if there is device-reserved metadata.

                        |<---sizeof(xdp_meta)--|
                        |                      |
                 struct xdp_meta               rx_desc->address
                        ^                      ^
                        |                      |
+----------+----------------------+------------+------+
| headroom |    custom metadata   |  reserved  | data |
+----------+----------------------+------------+------+
           ^                      ^            ^
           |                      |            |
    struct xdp_meta     xdp_buff->data_meta    xdp_buff->data
           |                      |
           |<---sizeof(xdp_meta)--|

Song Yoong Siang (2):
  doc: clarify XDP Rx metadata layout and bpf_xdp_adjust_meta usage
  selftests/bpf: Enhance XDP Rx Metadata Handling

 Documentation/networking/xdp-rx-metadata.rst  | 38 +++++++++++++++++++
 .../selftests/bpf/prog_tests/xdp_metadata.c   |  2 +-
 .../selftests/bpf/progs/xdp_hw_metadata.c     | 10 ++++-
 .../selftests/bpf/progs/xdp_metadata.c        |  8 +++-
 tools/testing/selftests/bpf/xdp_hw_metadata.c |  2 +-
 tools/testing/selftests/bpf/xdp_metadata.h    |  7 ++++
 6 files changed, 63 insertions(+), 4 deletions(-)

-- 
2.34.1

[PATCH bpf-next 1/2] doc: clarify XDP Rx metadata layout and bpf_xdp_adjust_meta usage

[Song Yoong Siang]

Expand the explanation of how METADATA_SIZE should be chosen to accommodate
both device-reserved and custom metadata. Additionally, add a diagram to
illustrate the calculation of the delta parameter for bpf_xdp_adjust_meta,
including alignment and size constraints.

These changes help users correctly allocate and access metadata in AF_XDP
use cases.

Signed-off-by: Song Yoong Siang <yoong.siang.song@intel.com>
---
 Documentation/networking/xdp-rx-metadata.rst | 38 ++++++++++++++++++++
 1 file changed, 38 insertions(+)

diff --git a/Documentation/networking/xdp-rx-metadata.rst b/Documentation/networking/xdp-rx-metadata.rst
index a6e0ece18be5..61418f533e0e 100644
--- a/Documentation/networking/xdp-rx-metadata.rst
+++ b/Documentation/networking/xdp-rx-metadata.rst
@@ -54,6 +54,19 @@ area in whichever format it chooses. Later consumers of the metadata
 will have to agree on the format by some out of band contract (like for
 the AF_XDP use case, see below).
 
+It is important to note that some devices may utilize the ``data_meta`` area for
+their own purposes. For example, the IGC device utilizes ``IGC_TS_HDR_LEN``
+bytes of the ``data_meta`` area for receiving hardware timestamps. Therefore,
+the XDP program should ensure that it does not overwrite any existing metadata.
+The metadata layout of such device is depicted below::
+
+  +----------+-----------------+--------------------------+------+
+  | headroom | custom metadata | device-reserved metadata | data |
+  +----------+-----------------+--------------------------+------+
+             ^                                            ^
+             |                                            |
+   xdp_buff->data_meta                              xdp_buff->data
+
 AF_XDP
 ======
 
@@ -76,6 +89,31 @@ Here is the ``AF_XDP`` consumer layout (note missing ``data_meta`` pointer)::
                                |
                         rx_desc->address
 
+It is crucial that the agreed ``METADATA_SIZE`` between the BPF program and the
+final consumer is sufficient to accommodate both device-reserved metadata and
+the data the BPF program needs to populate. When calling
+``bpf_xdp_adjust_meta``, the input parameter ``delta`` should be calculated as
+``METADATA_SIZE - (xdp_buff->data - xdp_buff->data_meta)``.
+
+The diagram below provides a visual representation of the calculation of
+``delta`` and the overall metadata layout::
+
+             |<-------------------METADATA_SIZE------------------->|
+  +----------+--------------------------+--------------------------+------+
+  | headroom |      custom metadata     | device-reserved metadata | data |
+  +----------+--------------------------+--------------------------+------+
+             ^                          ^                          ^
+             |                          |                          |
+  new xdp_buff->data_meta    old xdp_buff->data_meta        xdp_buff->data
+             |                          |
+             |<----------delta--------->|
+
+``bpf_xdp_adjust_meta`` ensures that ``METADATA_SIZE`` is aligned to 4 bytes,
+does not exceed 252 bytes, and leaves sufficient space for building the
+xdp_frame. If these conditions are not met, it returns a negative error. In this
+case, the BPF program should not proceed to populate data into the ``data_meta``
+area.
+
 XDP_PASS
 ========
 
-- 
2.34.1

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

[Song Yoong Siang]

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;
+	metalen_to_adjust = XDP_METADATA_SIZE - metalen_used;
+	if (metalen_to_adjust < (int)sizeof(struct xdp_meta)) {
+		__sync_add_and_fetch(&pkts_skip, 1);
+		return XDP_PASS;
+	}
+
+	err = bpf_xdp_adjust_meta(ctx, -metalen_to_adjust);
 	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..a0ba4ef4bbd8 100644
--- a/tools/testing/selftests/bpf/progs/xdp_metadata.c
+++ b/tools/testing/selftests/bpf/progs/xdp_metadata.c
@@ -37,6 +37,7 @@ extern int bpf_xdp_metadata_rx_vlan_tag(const struct xdp_md *ctx,
 SEC("xdp")
 int rx(struct xdp_md *ctx)
 {
+	int metalen_used, metalen_to_adjust;
 	void *data, *data_meta, *data_end;
 	struct ipv6hdr *ip6h = NULL;
 	struct ethhdr *eth = NULL;
@@ -73,7 +74,12 @@ int rx(struct xdp_md *ctx)
 
 	/* Reserve enough for all custom metadata. */
 
-	ret = bpf_xdp_adjust_meta(ctx, -(int)sizeof(struct xdp_meta));
+	metalen_used = ctx->data - ctx->data_meta;
+	metalen_to_adjust = XDP_METADATA_SIZE - metalen_used;
+	if (metalen_to_adjust < (int)sizeof(struct xdp_meta))
+		return XDP_DROP;
+
+	ret = bpf_xdp_adjust_meta(ctx, -metalen_to_adjust);
 	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)
-- 
2.34.1

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@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? 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.

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

[Song, Yoong Siang]

On Wednesday, July 2, 2025 12:31 AM, Stanislav Fomichev <stfomichev@gmail.com> 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

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

[Song, Yoong Siang]

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 <stfomichev@gmail.com>
>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?

Thanks & Regards
Siang

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

[Stanislav Fomichev]

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 <stfomichev@gmail.com>
> >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?

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

[Song, Yoong Siang]

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
><stfomichev@gmail.com>
>> >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?

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

[Stanislav Fomichev]

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
> ><stfomichev@gmail.com>
> >> >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!

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

[Song, Yoong Siang]

On Thursday, July 3, 2025 12:04 AM, Stanislav Fomichev <stfomichev@gmail.com> wrote:
>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
>> ><stfomichev@gmail.com>
>> >> >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!

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).

Re: [PATCH bpf-next 0/2] Clarify and Enhance XDP Rx Metadata Handling

[Jakub Kicinski]

On Tue,  1 Jul 2025 12:29:38 +0800 Song Yoong Siang wrote:
>                         |<---sizeof(xdp_meta)--|
>                         |                      |
>                  struct xdp_meta               rx_desc->address
>                         ^                      ^
>                         |                      |
> +----------+----------------------+------------+------+
> | headroom |    custom metadata   |  reserved  | data |
> +----------+----------------------+------------+------+
>            ^                      ^            ^
>            |                      |            |
>     struct xdp_meta     xdp_buff->data_meta    xdp_buff->data
>            |                      |
>            |<---sizeof(xdp_meta)--|

Huh. Did AF_XDP maintainers explicitly sign off on this or it's just how
IGC implementation works and nobody noticed?

For normal XDP my understanding is that its the driver's responsibility
to move the "reserved" stuff out of place before presenting the frame to
program.

RE: [PATCH bpf-next 0/2] Clarify and Enhance XDP Rx Metadata Handling

[Song, Yoong Siang]

On Tuesday, July 8, 2025 4:55 AM, Jakub Kicinski <kuba@kernel.org> wrote:
>On Tue,  1 Jul 2025 12:29:38 +0800 Song Yoong Siang wrote:
>>                         |<---sizeof(xdp_meta)--|
>>                         |                      |
>>                  struct xdp_meta               rx_desc->address
>>                         ^                      ^
>>                         |                      |
>> +----------+----------------------+------------+------+
>> | headroom |    custom metadata   |  reserved  | data |
>> +----------+----------------------+------------+------+
>>            ^                      ^            ^
>>            |                      |            |
>>     struct xdp_meta     xdp_buff->data_meta    xdp_buff->data
>>            |                      |
>>            |<---sizeof(xdp_meta)--|
>
>Huh. Did AF_XDP maintainers explicitly sign off on this or it's just how
>IGC implementation works and nobody noticed?
>
Previously,  IGC do copy out the Rx hwts from metadata area,
so no problem when implementing XDP Rx metadata.

After that, net_device_ops.ndo_get_tstamp() is added into IGC to
support timestamping from both free-running clock and adjustable clock.
The 2 timers are stored in the metadata area, thus causing the issue.

>For normal XDP my understanding is that its the driver's responsibility
>to move the "reserved" stuff out of place before presenting the frame to
>program.

Is it means that driver needs to move out the "reserved" stuff before XDP program
and then move back the stuff after XDP program for certain situation, like XDP_PASS?

IMHO, if driver is allowed to use some portion of the metadata area, then
the packet processing will be more efficiency and also align with the "zero-copy" idea.

Any thoughts?
  

Re: [PATCH bpf-next 0/2] Clarify and Enhance XDP Rx Metadata Handling

[Jakub Kicinski]

On Tue, 8 Jul 2025 01:34:13 +0000 Song, Yoong Siang wrote:
> >For normal XDP my understanding is that its the driver's responsibility
> >to move the "reserved" stuff out of place before presenting the frame to
> >program.  
> 
> Is it means that driver needs to move out the "reserved" stuff before XDP program
> and then move back the stuff after XDP program for certain situation, like XDP_PASS?

Why would the driver need to move it back?
On XDP_PASS an skb is constructed, so the metadata should 
be transferred to the skb. There is no need to copy it back
as a prepend.

RE: [PATCH bpf-next 0/2] Clarify and Enhance XDP Rx Metadata Handling

[Song, Yoong Siang]

On Tuesday, July 8, 2025 9:45 AM, Jakub Kicinski <kuba@kernel.org> wrote:
>On Tue, 8 Jul 2025 01:34:13 +0000 Song, Yoong Siang wrote:
>> >For normal XDP my understanding is that its the driver's responsibility
>> >to move the "reserved" stuff out of place before presenting the frame to
>> >program.
>>
>> Is it means that driver needs to move out the "reserved" stuff before XDP program
>> and then move back the stuff after XDP program for certain situation, like
>XDP_PASS?
>
>Why would the driver need to move it back?
>On XDP_PASS an skb is constructed, so the metadata should
>be transferred to the skb. There is no need to copy it back
>as a prepend.

I said so because I thought need to put back the timestamp
as prepend and then point skb_shared_hwtstamps.netdev_data to it
to support the ndo_get_tstamp().

I haven't study the code flow in detail, so I might be missing something.

Re: [PATCH bpf-next 0/2] Clarify and Enhance XDP Rx Metadata Handling

[Jakub Kicinski]

On Tue, 8 Jul 2025 02:06:11 +0000 Song, Yoong Siang wrote:
>> Why would the driver need to move it back?
>> On XDP_PASS an skb is constructed, so the metadata should
>> be transferred to the skb. There is no need to copy it back
>> as a prepend.  
> 
> I said so because I thought need to put back the timestamp
> as prepend and then point skb_shared_hwtstamps.netdev_data to it
> to support the ndo_get_tstamp().

No need, the timestamps are set in shared info directly.
There are multiple drivers which use the metadata prepend
method, so I'm pretty sure it should work.

RE: [PATCH bpf-next 0/2] Clarify and Enhance XDP Rx Metadata Handling

[Song, Yoong Siang]

On Tuesday, July 8, 2025 10:18 AM, Jakub Kicinski <kuba@kernel.org> wrote:
>On Tue, 8 Jul 2025 02:06:11 +0000 Song, Yoong Siang wrote:
>>> Why would the driver need to move it back?
>>> On XDP_PASS an skb is constructed, so the metadata should
>>> be transferred to the skb. There is no need to copy it back
>>> as a prepend.
>>
>> I said so because I thought need to put back the timestamp
>> as prepend and then point skb_shared_hwtstamps.netdev_data to it
>> to support the ndo_get_tstamp().
>
>No need, the timestamps are set in shared info directly.
>There are multiple drivers which use the metadata prepend
>method, so I'm pretty sure it should work.

Thanks for pointing me in the right direction.
I'll proceed with updating the IGC driver and conduct tests
to ensure everything works as expected.