- Improve performace for writer-only BPF users.

Linux and Solaris (at least OpenSolaris) has PF_PACKET socket families to send
raw ethernet frames. The only FreeBSD interface that can be used to send raw frames
is BPF. As a result, many programs like cdpd, lldpd, various dhcp stuff uses
BPF only to send data. This leads us to the situation when software like cdpd,
being run on high-traffic-volume interface significantly reduces overall performance
since we have to acquire additional locks for every packet.

Here we add sysctl that changes BPF behavior in the following way:
If program came and opens BPF socket without explicitly specifyin read filter we
assume it to be write-only and add it to special writer-only per-interface list.
This makes bpf_peers_present() return 0, so no additional overhead is introduced.
After filter is supplied, descriptor is added to original per-interface list permitting
packets to be captured.

Unfortunately, pcap_open_live() sets catch-all filter itself for the purpose of
setting snap length.

Fortunately, most programs explicitly sets (event catch-all) filter after that.
tcpdump(1) is a good example.

So a bit hackis approach is taken: we upgrade description only after second
BIOCSETF is received.

Sysctl is named net.bpf.optimize_writers and is turned off by default.

- While here, document all sysctl variables in bpf.4

Sponsored by Yandex LLC

Reviewed by:    glebius (previous version)
Reviewed by:    silence on -net@
Approved by:    (mentor)

MFC after:      4 weeks

1 files changed, 1 insertions, 0 deletions

diff --git a/sys/net/bpf.h b/sys/net/bpf.h
index a347453..0cd9656 100644
--- a/sys/net/bpf.h
+++ b/sys/net/bpf.h
@@ -1104,6 +1104,7 @@ struct bpf_if {
 	u_int bif_hdrlen;		/* length of link header */
 	struct ifnet *bif_ifp;		/* corresponding interface */
 	struct rwlock bif_lock;		/* interface lock */
+	LIST_HEAD(, bpf_d)	bif_wlist;	/* writer-only list */
 #endif
 };