On 05/10/2022 02.25, Martin KaFai Lau wrote:
For rx hwtimestamp, the value could be just 0 if a specific hw/driver cannot provide it for all packets while some other hw can.
Keep in mind that we want to avoid having to write a (64-bit) zero into the metadata for rx_hwtimestamp, for every packet that doesn't carry a timestamp. It essentially reverts back to clearing memory like with SKBs, due to performance overhead we don't want to go that path again! There are multiple ways to avoid having to zero init the memory. In this patchset I have choosen have the traditional approach of flags (u32) approach located in xdp_hints_common, e.g. setting a flag if the field is valid (p.s. John Fastabend convinced me of this approach ;-)). But COMBINED with: some BTF ID layouts doesn't contain some fields e.g. the rx_timestamp, thus the code have no reason to query those flag fields. I am intrigued to find a way to leverage bpf_core_field_exists() some more (as proposed by Stanislav). (As this can allow for dead-code elimination). --Jesper
