[xdp-hints] Re: [RFC bpf-next v2 10/14] ice: Support rx timestamp metadata for xdp

[Larysa Zaremba <larysa.zaremba@intel.com>]

On Thu, Nov 03, 2022 at 08:25:28PM -0700, Stanislav Fomichev wrote:
> +			/* if (r5 == NULL) return; */
> +			BPF_JMP_IMM(BPF_JNE, BPF_REG_5, 0, S16_MAX),

S16_MAX jump crashes my system and I do not see such jumps used very often
in bpf code found in-tree, setting a fixed jump length worked for me.
Also, I think BPF_JEQ is a correct condition in this case, not BPF_JNE.

But the main reason for my reply is that I have implemented RX hash hint
for ice both as unrolled bpf code and with BPF_EMIT_CALL [0].
Both bpf_xdp_metadata_rx_hash() and bpf_xdp_metadata_rx_hash_supported() 
are implemented in those 2 ways.

RX hash is the easiest hint to read, so performance difference
should be more visible than when reading timestapm.

Counting packets in an rxdrop XDP program on a single queue
gave me the following numbers:

- unrolled:		41264360 pps
- BPF_EMIT_CALL:	40370651 pps

So, reading a single hint in an unrolled way instead of calling 2 driver
functions in a row, gives us a 2.2% performance boost.
Surely, the difference will increase, if we read more than a single hint.
Therefore, it would be great to implement at least some simple hints
functions as unrolled.

[0] https://github.com/walking-machine/linux/tree/ice-kfunc-hints-clean