On 05/10/2022 03.02, Stanislav Fomichev wrote:
On Tue, Oct 4, 2022 at 5:59 PM Jakub Kicinski <kuba@xxxxxxxxxx> 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)..
On the technical side we do have the ifindex that can be passed along
which is currently used for getting XDP hardware offloading to work.
But last time I tried this, I failed due to BPF tail call maps.
(It's not going to fly for other reasons, see redirect below).
+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?
Exactly the problem. With XDP redirect the "remote" target device also
need to interpret this metadata layout. For RX-side we have the
immediate case with redirecting into a veth device. For future TX-side
this is likely the same kind of issue, but I hope if we can solve this
for veth redirect use-case, this will keep us future proof.
For veth use-case I hope that we can use same trick as
bpf_core_field_exists() to do dead-code elimination based on if a device
driver is loaded on the system like this pseudo code:
if (bpf_core_type_id_kernel(struct xdp_hints_i40e_timestamp)) {
/* check id + extract timestamp */
}
if (bpf_core_type_id_kernel(struct xdp_hints_ixgbe_timestamp)) {
/* check id + extract timestamp */
}
If the given device drives doesn't exist on the system, I assume
bpf_core_type_id_kernel() will return 0 at libbpf relocation/load-time,
and thus this should cause dead-code elimination. Should work today AFAIK?
--Jesper
