bpf/xdp: optimize bpf_xdp_pointer to avoid reading sinfo

bpf/xdp: optimize bpf_xdp_pointer to avoid reading sinfo

Currently we observed a significant performance degradation in
samples/bpf xdp1 and xdp2, due XDP multibuffer "xdp.frags" handling,
added in commit 772251742262 ("samples/bpf: fixup some tools to be able
to support xdp multibuffer").

This patch reduce the overhead by avoiding to read/load shared_info
(sinfo) memory area, when XDP packet don't have any frags. This improves
performance because sinfo is located in another cacheline.

Function bpf_xdp_pointer() is used by BPF helpers bpf_xdp_load_bytes()
and bpf_xdp_store_bytes(). As a help to reviewers, xdp_get_buff_len() can
potentially access sinfo, but it uses xdp_buff_has_frags() flags bit check
to avoid accessing sinfo in no-frags case.

The likely/unlikely instrumentation lays out asm code such that sinfo
access isn't interleaved with no-frags case (checked on GCC 12.2.1-4).
The generated asm code is more compact towards the no-frags case.

The BPF kfunc bpf_dynptr_slice() also use bpf_xdp_pointer(). Thus, it
should also take effect for that.

Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
Acked-by: Lorenzo Bianconi <lorenzo@kernel.org>
Acked-by: Toke Høiland-Jørgensen <toke@redhat.com>
Link: https://lore.kernel.org/r/168563651438.3436004.17735707525651776648.stgit@firesoul
Signed-off-by: Alexei Starovoitov <ast@kernel.org>