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 7722517422 ("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>

net/core/filter.c

@@ -3948,20 +3948,21 @@ void bpf_xdp_copy_buf(struct xdp_buff *xdp, unsigned long off,
 
 void *bpf_xdp_pointer(struct xdp_buff *xdp, u32 offset, u32 len)
 {
-	struct skb_shared_info *sinfo = xdp_get_shared_info_from_buff(xdp);
 	u32 size = xdp->data_end - xdp->data;
+	struct skb_shared_info *sinfo;
 	void *addr = xdp->data;
 	int i;
 
 	if (unlikely(offset > 0xffff || len > 0xffff))
 		return ERR_PTR(-EFAULT);
 
-	if (offset + len > xdp_get_buff_len(xdp))
+	if (unlikely(offset + len > xdp_get_buff_len(xdp)))
 		return ERR_PTR(-EINVAL);
 
-	if (offset < size) /* linear area */
+	if (likely(offset < size)) /* linear area */
 		goto out;
 
+	sinfo = xdp_get_shared_info_from_buff(xdp);
 	offset -= size;
 	for (i = 0; i < sinfo->nr_frags; i++) { /* paged area */
 		u32 frag_size = skb_frag_size(&sinfo->frags[i]);