Simplify the way of attaching IPv6 link-layer header.

Problem description:
How do we currently perform layer 2 resolution and header imposition:

For IPv4 we have the following chain:
  ip_output() -> (ether|atm|whatever)_output() -> arpresolve()

Lookup is done in proper place (link-layer output routine) and it is possible
  to provide cached lle data.

For IPv6 situation is more complex:
  ip6_output() -> nd6_output() -> nd6_output_ifp() -> (whatever)_output() ->
    nd6_storelladdr()

We have ip6_ouput() which calls nd6_output() instead of link output routine.
nd6_output() does the following:
  * checks if lle exists, creates it if needed (similar to arpresolve())
  * performes lle state transitions (similar to arpresolve())
  * calls nd6_output_ifp() which pushes packets to link output routine along
    with running SeND/MAC hooks regardless of lle state
    (e.g. works as run-hooks placeholder).

After that, iface output routine like ether_output() calls nd6_storelladdr()
  which performs lle lookup once again.

As a result, we perform lookup twice for each outgoing packet for most types
  of interfaces. We also need to maintain runtime-checked table of 'nd6-free'
  interfaces (see nd6_need_cache()).

Fix this behavior by eliminating first ND lookup. To be more specific:
  * make all nd6_output() consumers use nd6_output_ifp() instead
  * rename nd6_output[_slow]() to nd6_resolve_[slow]()
  * convert nd6_resolve() and nd6_resolve_slow() to arpresolve() semantics,
    e.g. copy L2 address to buffer instead of pushing packet towards lower
    layers
  * Make all nd6_storelladdr() users use nd6_resolve()
  * eliminate nd6_storelladdr()

The resulting callchain is the following:
  ip6_output() -> nd6_output_ifp() -> (whatever)_output() -> nd6_resolve()

Error handling:
Currently sending packet to non-existing la results in ip6_<output|forward>
  -> nd6_output() -> nd6_output _lle() which returns 0.
In new scenario packet is propagated to <ether|whatever>_output() ->
  nd6_resolve() which will return EWOULDBLOCK, and that result
  will be converted to 0.

(And EWOULDBLOCK is actually used by IB/TOE code).

Sponsored by:		Yandex LLC
Differential Revision:	https://reviews.freebsd.org/D1469

1 files changed, 12 insertions, 10 deletions

diff --git a/sys/net/if_arcsubr.c b/sys/net/if_arcsubr.c
index d6724cf..d59c4a0 100644
--- a/sys/net/if_arcsubr.c
+++ b/sys/net/if_arcsubr.c
@@ -103,8 +103,8 @@ arc_output(struct ifnet *ifp, struct mbuf *m, const struct sockaddr *dst,
 	u_int8_t		atype, adst;
 	int			loop_copy = 0;
 	int			isphds;
-#ifdef INET
-	int			is_gw;
+#if defined(INET) || defined(INET6)
+	int			is_gw = 0;
 #endif
 
 	if (!((ifp->if_flags & IFF_UP) &&
@@ -112,6 +112,11 @@ arc_output(struct ifnet *ifp, struct mbuf *m, const struct sockaddr *dst,
 		return(ENETDOWN); /* m, m1 aren't initialized yet */
 
 	error = 0;
+#if defined(INET) || defined(INET6)
+	if (ro != NULL && ro->ro_rt != NULL &&
+	    (ro->ro_rt->rt_flags & RTF_GATEWAY) != 0)
+		is_gw = 1;
+#endif
 
 	switch (dst->sa_family) {
 #ifdef INET
@@ -125,10 +130,6 @@ arc_output(struct ifnet *ifp, struct mbuf *m, const struct sockaddr *dst,
 		else if (ifp->if_flags & IFF_NOARP)
 			adst = ntohl(SIN(dst)->sin_addr.s_addr) & 0xFF;
 		else {
-			is_gw = 0;
-			if (ro != NULL && ro->ro_rt != NULL &&
-			    (ro->ro_rt->rt_flags & RTF_GATEWAY) != 0)
-				is_gw = 1;
 			error = arpresolve(ifp, is_gw, m, dst, &adst, NULL);
 			if (error)
 				return (error == EWOULDBLOCK ? 0 : error);
@@ -169,10 +170,11 @@ arc_output(struct ifnet *ifp, struct mbuf *m, const struct sockaddr *dst,
 	case AF_INET6:
 		if ((m->m_flags & M_MCAST) != 0)
 			adst = arcbroadcastaddr; /* ARCnet broadcast address */
-		else
-			error = nd6_storelladdr(ifp, m, dst, (u_char *)&adst, NULL);
-		if (error)
-			return (error);
+		else {
+			error = nd6_resolve(ifp, is_gw, m, dst, &adst, NULL);
+			if (error != 0)
+				return (error == EWOULDBLOCK ? 0 : error);
+		}
 		atype = ARCTYPE_INET6;
 		break;
 #endif