[xdp-hints] Re: [PATCH RFCv2 bpf-next 00/18] XDP-hints: XDP gaining access to HW offload hints via BTF

[Stanislav Fomichev <sdf@google.com>]

On Tue, Oct 4, 2022 at 5:59 PM Jakub Kicinski <kuba@kernel.org> wrote:
>
> On Tue, 4 Oct 2022 17:25:51 -0700 Martin KaFai Lau wrote:
> > A intentionally wild question, what does it take for the driver to return the
> > hints.  Is the rx_desc and rx_queue enough?  When the xdp prog is calling a
> > kfunc/bpf-helper, like 'hwtstamp = bpf_xdp_get_hwtstamp()', can the driver
> > replace it with some inline bpf code (like how the inline code is generated for
> > the map_lookup helper).  The xdp prog can then store the hwstamp in the meta
> > area in any layout it wants.
>
> Since you mentioned it... FWIW that was always my preference rather than
> the BTF magic :)  The jited image would have to be per-driver like we
> do for BPF offload but that's easy to do from the technical
> perspective (I doubt many deployments bind the same prog to multiple
> HW devices)..

+1, sounds like a good alternative (got your reply while typing)
I'm not too versed in the rx_desc/rx_queue area, but seems like worst
case that bpf_xdp_get_hwtstamp can probably receive a xdp_md ctx and
parse it out from the pre-populated metadata?

Btw, do we also need to think about the redirect case? What happens
when I redirect one frame from a device A with one metadata format to
a device B with another?