Re: [PATCH bpf-next,v3 1/2] doc: enhance explanation of XDP Rx metadata layout and METADATA_SIZE

[Daniel Borkmann]

On 7/2/25 6:57 PM, Song Yoong Siang wrote:
[...]
> +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

Imho, this section is misleading to developers. Suppose you're a XDP program writer
and you want to implement a generic native BPF program (independent of the underlying
NIC). Does this mean, the expectation is to dig into driver code to gather whether
or not a driver is prepopulating and how much of it? What are the implications if the
data is overwritten? For example, in Cilium today we use the buffer described here
as device-reserved metadata and override it. How will users know what breaks?