Add description for what each of the parameters are to the cam_sim_alloc
call. Add some additional context for the mtx and queue parameters to
explain what special values passed in mean.

MFC After:		3 days
Reviewed by:		mav@
Sponsored by:		Netflix
Differential Revision:	https://reviews.freebsd.org/D30115

DXR maintains compressed lookup structures with a trivial search
procedure.  A two-stage trie is indexed by the more significant bits of
the search key (IPv4 address), while the remaining bits are used for
finding the next hop in a sorted array.  The tradeoff between memory
footprint and search speed depends on the split between the trie and
the remaining binary search.  The default of 20 bits of the key being
used for trie indexing yields good performance (see below) with
footprints of around 2.5 Bytes per prefix with current BGP snapshots.

Rebuilding lookup structures takes some time, which is compensated for by
batching several RIB change requests into a single FIB update, i.e. FIB
synchronization with the RIB may be delayed for a fraction of a second.
RIB to FIB synchronization, next-hop table housekeeping, and lockless
lookup capability is provided by the FIB_ALGO infrastructure.

DXR works well on modern CPUs with several MBytes of caches, especially
in VMs, where is outperforms other currently available IPv4 FIB
algorithms by a large margin.

Synthetic single-thread LPM throughput test method:

kldload test_lookup; kldload dpdk_lpm4; kldload fib_dxr
sysctl net.route.test.run_lps_rnd=N
sysctl net.route.test.run_lps_seq=N

where N is the number of randomly generated keys (IPv4 addresses) which
should be chosen so that each test iteration runs for several seconds.

Each reported score represents the best of three runs, in million
lookups per second (MLPS), for two bechmarks (RND & SEQ) with two FIBs:

host: single interface address, local subnet route + default route
BGP: snapshot from linx.routeviews.org, 887957 prefixes, 496 next hops

Bhyve VM on an Intel(R) Xeon(R) CPU E5-2670 0 @ 2.60 GHz:
inet.algo         host, RND    host, SEQ    BGP, RND    BGP, SEQ
bsearch4             40.6         20.2         N/A         N/A
radix4                7.8          3.8         1.2         0.6
radix4_lockless      18.0          9.0         1.6         0.8
dpdk_lpm4            14.4          5.0        14.6         5.0
dxr                  70.3         34.7        43.0        19.5

Intel(R) Core(TM) i5-5300U CPU @ 2.30 GHz:
inet.algo         host, RND    host, SEQ    BGP, RND    BGP, SEQ
bsearch4             47.0         23.1         N/A         N/A
radix4                8.5          4.2         1.9         1.0
radix4_lockless      19.2          9.5         2.5         1.2
dpdk_lpm4            31.2          9.4        31.6         9.3
dxr                  84.9         41.4        51.7        23.6

Intel(R) Core(TM) i7-4771 CPU @ 3.50 GHz:
inet.algo         host, RND    host, SEQ    BGP, RND    BGP, SEQ
bsearch4             59.5         29.4         N/A         N/A
radix4               10.8          5.5         2.5         1.3
radix4_lockless      24.7         12.0         3.1         1.6
dpdk_lpm4            29.1          9.0        30.2         9.1
dxr                 101.3         49.9        69.8        32.5

AMD Ryzen 7 3700X 8-Core Processor @ 3.60 GHz:
inet.algo         host, RND    host, SEQ    BGP, RND    BGP, SEQ
bsearch4             70.8         35.4         N/A         N/A
radix4               14.4          7.2         2.8         1.4
radix4_lockless      30.2         15.1         3.7         1.8
dpdk_lpm4            29.9          9.0        30.0         8.9
dxr                 163.3         81.5        99.5        44.4

AMD Ryzen 5 5600X 6-Core Processor @ 3.70 GHz:
inet.algo         host, RND    host, SEQ    BGP, RND    BGP, SEQ
bsearch4             93.6         46.7         N/A         N/A
radix4               18.9          9.3         4.3         2.1
radix4_lockless      37.2         18.6         5.3         2.7
dpdk_lpm4            51.8         15.1        51.6        14.9
dxr                 218.2        103.3       114.0        49.0

Reviewed by:	melifaro
MFC after:	1 week
Differential Revision: https://reviews.freebsd.org/D29821

Add a LPS benchmark variant which introduces artificial dependencies
between successive lookups. While here, instead of writing the results
from the lookups to a huge array, add them to an accumulator, in a more
lightweight attempt at preventing the CPU's OOO machinery from
discarding the lookup results if they would be completely unused.

net.route.test.run_lps_rnd measures LPS throughput with independent
uniformly random keys

net.route.test.run_lps_seq measures LPS throughput with uniformly
random keys with artificial interdependencies
Reviewed by:	melifaro
MFC after:	7 days
Differential Revision: https://reviews.freebsd.org/D30096