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, 2 insertions, 2 deletions

diff --git a/sys/netinet6/ip6_output.c b/sys/netinet6/ip6_output.c
index 09d0925..181340f 100644
--- a/sys/netinet6/ip6_output.c
+++ b/sys/netinet6/ip6_output.c
@@ -935,7 +935,7 @@ passout:
 			    m->m_pkthdr.len);
 			ifa_free(&ia6->ia_ifa);
 		}
-		error = nd6_output(ifp, origifp, m, dst, ro->ro_rt);
+		error = nd6_output_ifp(ifp, origifp, m, dst);
 		goto done;
 	}
 
@@ -1034,7 +1034,7 @@ sendorfree:
 				counter_u64_add(ia->ia_ifa.ifa_obytes,
 				    m->m_pkthdr.len);
 			}
-			error = nd6_output(ifp, origifp, m, dst, ro->ro_rt);
+			error = nd6_output_ifp(ifp, origifp, m, dst);
 		} else
 			m_freem(m);
 	}