General documentation updates

- We now have several developer-focused docs, so create an index page
  for them.
- Rework several docs to fit into the index structure.
- Experiment with code highlighting; tweak the CSS slightly to make
  it slightly nicer to look at.

Change-Id: I4185a18f84fa0764745ca7a3148276064a3155c6
Signed-off-by: Chris Luke <chrisy@flirble.org>

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+Multicore support for ACL plugin    {#acl_multicore}
+================================
+
+This captures some considerations and design decisions that I have made,
+both for my own memory later on ("what the hell was I thinking?!?"),
+and for anyone interested to criticize/improve/hack on this code.
+
+One of the factors taken into account while making these decisions,
+was the relative emphasis on the multi-thread vs. single-thread
+use cases: the latter is the vastly more prevalent. But,
+one can not optimize the single-thread performance without
+having a functioning code for multi-thread.
+
+stateless ACLs
+==============
+
+The stateless trivially parallelizes, and the only potential for the
+race between the different threads is during the reconfiguration,
+at the time of replacing the old ACL being checked, with
+the new ACL.
+
+In case an acl_add_replace is being used to replace the rules
+within the existing entry, a reallocation of `am->acls[X].rules`
+vector will happen and potentially a change in count.
+
+acl_match_5tuple() has the following code:
+
+```{.c}
+  a = am->acls + acl_index;
+  for (i = 0; i < a->count; i++)
+    {
+      r = a->rules + i;
+     . . .
+```
+
+Ideally we should be immune from a->rules changing,
+but the problem arises if the count changes in flight,
+and the new ruleset is smaller - then we will attempt
+to "match" against the free memory.
+
+This can(?) be solved by replacing the for() with while(),
+so the comparison happens at each iteration.
+
+full_acl_match_5tuple(), which iterates over the list
+of ACLs, is a bit less immune, since it takes the pointer
+to the vector to iterate and keeps a local copy of
+that pointer.
+
+This race can be solved by checking the
+current pointer to the vector with the source pointer,
+and seeing if there is an (unlikely) change, and if
+there is, return the "deny" action, or, better,
+restart the check.
+
+Since the check reloads the ACL list on a per-packet basis,
+there is only a window of opportunity of one packet to
+"match" packet against an incorrect rule set.
+The workers also do not change anything, only read.
+Therefore, it looks like building special structures
+to ensure that it does not happen at all might be not
+worth it.
+
+At least not until we have a unit-test able to
+reliably catch this condition and test that
+the measures applied are effective. Adding the code
+which is not possible to exercise is worse than
+not adding any code at all.
+
+So, I opt for "do-nothing" here for the moment.
+
+reflexive ACLs: single-thread
+=============================
+
+Before we talk multi-thread, is worth revisiting the
+design of the reflexive ACLs in the plugin, and
+the history of their evolution.
+
+The very first version of the ACL plugin, shipped in
+1701, mostly did the job using the existing components
+and gluing them together. Because it needed to work
+in bridged forwarding path only, using L2 classifier
+as an insertion point appeared natural, also L2 classifier,
+being a table with sessions, seemed like a good place
+to hold the sessions.
+
+So, the original design had two conceptual nodes:
+one, pointed by the next_miss from the L2 classifier table,
+was checking the actual ACL, and inserting session into
+the L2 classifier table, and the other one, pointed
+to by the next_match within the specific session rule,
+was checking the existing session. The timing out
+of the existing connections was done in the datapath,
+by periodically calling the aging function.
+
+This decision to use the existing components,
+with its attrativeness, did bring a few limitations as well:
+
+* L2 classifier is a simple mask-and-value match, with
+a fixed mask across the table. So, sanely supporting IPv6
+packets with extension headers in that framework was impossible.
+
+* There is no way to get a backpressure from L2 classifier
+depending on memory usage. When it runs out of memory,
+it simply crashes the box. When it runs out of memory ?
+We don't really know. Depends on how it allocates it.
+
+* Since we need to match the *reflected* traffic,
+we had to create *two* full session entries
+in two different directions, which is quite wasteful memory-wise.
+
+* (showstopper): the L2 classifier runs only in
+the bridged data path, so supporting routed data path
+would require creating something else entirely different,
+which would mean much more headaches support-wise going forward.
+
+Because of that, I have moved to a different model of
+creating a session-5-tuple from the packet data - once,
+and then doing all the matching just on that 5-tuple.
+
+This has allowed to add support for skipping IPv6 extension headers.
+
+Also, this new version started to store the sessions in a dedicated
+bihash-per-interface, with the session key data being
+aligned for the ingress packets, and being mirrored for the
+egress packets. This allows of significant savings in memory,
+because now we need to keep only one copy of the session table per
+interface instead of two, and also to only have ONE node for all the lookups,
+(L2/L3 path, in/out, IPv4/IPv6) - significantly reducing the code complexity.
+
+Unfortunately, bihash still has the "lack of backpressure" problem,
+in a sense that if you try to insert too many entries and run out
+of memory in the heap you supplied, you get a crash.
+
+To somewhat workaround against that, there is a "maximum tested number of sessions"
+value, which tracks the currently inserted sessions in the bihash,
+and if this number is being approached, a more aggressive cleanup
+can happen. If this number is reached, two behaviors are possible:
+
+* attempt to do the stateless ACL matching and permit the packet
+  if it succeeds
+
+* deny the packet
+
+Currently I have opted for a second one, since it allows for
+a better defined behavior, and if you have to permit
+the traffic in both directions, why using stateful anyway ?
+
+In order to be able to do the cleanup, we need to discriminate between
+the session types, with each session type having its own idle timeout.
+In order to do that, we keep three lists, defined in enum acl_timeout_e:
+ACL_TIMEOUT_UDP_IDLE, ACL_TIMEOUT_TCP_IDLE, ACL_TIMEOUT_TCP_TRANSIENT.
+
+The first one is hopefully obvious - it is just all UDP connections.
+They have an idle timeout of 600 seconds.
+
+The second and third is a bit more subtle. TCP is a complicated protocol,
+and we need to tread the fine line between doing too little and doing
+too much, and triggering the potential compatibility issues because of
+being a "middlebox".
+
+I decided to split the TCP connections into two classes:
+established, and everything else. "Established", means we have seen
+the SYN and ACK from both sides (with PUSH obviously masked out).
+This is the "active" state of any TCP connection and we would like
+to ensure we do not screw it up. So, the connections in this state
+have the default idle timer of 24 hours.
+
+All the rest of the connections have the idle timeout of 2 minutes,
+(inspired by an old value of MSL) and based on the observation
+that the states this class represent are usually very short lived.
+
+Once we have these three baskets of connections, it is trivial to
+imagine a simple cleanup mechanism to deal with this: take a
+TCP transient connection that has been hanging around.
+
+It is debatable whether we want to do discrimination between the
+different TCP transient connections. Assuming we do FIFO (and
+the lists allow us to do just that), it means a given connection
+on the head of the list has been hanging around for longest.
+Thus, if we are short on resources, we might just go ahead and
+reuse it within the datapath.
+
+This is where we are slowly approaching the question
+"Why in the world have not you used timer wheel or such ?"
+
+The answer is simple: within the above constraints, it does
+not buy me much.
+
+Also, timer wheel creates a leaky abstraction with a difficult
+to manage corner case. Which corner case ?
+
+We have a set of objects (sessions) with an event that may
+or may not happen (idle timeout timer firing), and a
+necessity to reset the idle timeout when there is
+activity on the session.
+
+In the worst case, where we had a 10000 of one-packet
+UDP sessions just created 10 minutes ago, we would need
+to deal with a spike of 10000 expired timers.
+
+Of course, if we have the active traffic on all
+of these 10000 connections, then we will not have
+to deal with that ? Right, but we will still have to deal
+with canceling and requeueing the timers.
+
+In the best possible case, requeueing a timer is
+going to be something along the lines of a linked-list
+removal and reinsertion.
+
+However, keep in mind we already need to classify the
+connections for reuse, so therefore we already have
+the linked lists!
+
+And if we just check these linked lists periodically in
+a FIFO fashion, we can get away with a very simple per-packet operation:
+writing back the timestamp of "now" into the connection structure.
+
+Then rather than requeueing the list on a per-packet or per-frame
+basis, we can defer this action until the time this session
+appears on the head of the FIFO list, and the cleaning
+routine makes the decision about whether to discard
+the session (because the interval since last activity is bigger
+than the idle timeout), or to requeue the session back to
+the end of the list (because the last activity was less
+than idle timeout ago).
+
+So, rather than using the timers, we can simply reuse our classification
+FIFOs, with the following heuristic: do not look at the session that was
+enqueued at time X until X+session_timeout. If we enqueue the sessions
+in the order of their initial activity, then we can simply use enqueue
+timestamp of the head session as a decision criterion for when we need
+to get back at looking at it for the timeout purposes.
+
+Since the number of FIFOs is small, we get a slightly worse check
+performance than with timers, but still O(1).
+
+We seemingly do quite a few "useless" operations of requeueing the items
+back to the tail of the list - but, these are the operations we do not
+have to do in the active data path, so overall it is a win.
+
+(Diversion: I believe this problem is congruent to poll vs. epoll or
+events vs. threads, some reading on this subject:
+http://web.archive.org/web/20120225022154/http://sheddingbikes.com/posts/1280829388.html)
+
+We can also can run a TCP-like scheme for adaptively changing
+the wait period in the routine that deals with the connection timeouts:
+we can attempt to check the connections a couple of times per second
+(same as we would advance the timer wheel), and then if we have requeued
+close to a max-per-quantum number of connections, we can half the waiting
+interval, and if we did not requeue any, we can slowly increment the waiting
+interval - which at a steady state should stabilize similar to what the TCP rate
+does.
+
+reflexive ACLs: multi-thread
+=============================
+
+The single-threaded implementation in 1704 used a separate "cleaner" process
+to deal with the timing out of the connections.
+It is all good and great when you know that there is only a single core
+to run everything on, but the existence of the lists proves to be
+a massive difficulty when it comes to operating from multiple threads.
+
+Initial study shows that with a few assumptions (e.g. that the cleaner running in main thread
+and the worker have a demarcation point in time where either one or the other one touches
+the session in the list) it might be possible to make it work, but the resulting
+trickiness of doing it neatly with all the corner cases is quite large.
+
+So, for the multi-threaded scenario, we need to move the connection
+aging back to the same CPU as its creation.
+
+Luckily we can do this with the help of the interrupts.
+
+So, the design is as follows: the aging thread (acl_fa_session_cleaner_process)
+periodically fires the interrupts to the workers interrupt nodes (acl_fa_worker_session_cleaner_process_node.index),
+using vlib_node_set_interrupt_pending(), and
+the interrupt node acl_fa_worker_conn_cleaner_process() calls acl_fa_check_idle_sessions()
+which does the actual job of advancing the lists. And within the actual datapath the only thing we will be
+doing is putting the items onto FIFO, and updating the last active time on the existing connection.
+
+The one "delicate" part is that the worker for one leg of the connection might be different from
+the worker of another leg of the connection - but, even if the "owner" tries to free the connection,
+nothing terrible can happen - worst case the element of the pool (which is nominally free for a short period)
+will get the timestamp updated - same thing about the TCP flags seen.
+
+A slightly trickier issue arises when the packet initially seen by one worker (thus owned by that worker),
+and the return packet processed by another worker, and as a result changes the
+the class of the connection (e.g. becomes TCP_ESTABLISHED from TCP_TRANSIENT or vice versa).
+If the class changes from one with the shorter idle time to the one with the longer idle time,
+then unless we are in the starvation mode where the transient connections are recycled,
+we can simply do nothing and let the normal requeue mechanism kick in. If the class changes from the longer idle
+timer to the shorter idle timer, then we risk keeping the connection around for longer than needed, which
+will affect the resource usage.
+
+One solution to that is to have NxN ring buffers (where N is the number of workers), such that the non-owner
+can signal to the owner the connection# that needs to be requeued out of order.
+
+A simpler solution though, is to ensure that each FIFO's period is equal to that of a shortest timer.
+This way the resource starvation problem is taken care of, at an expense of some additional work.
+
+This all looks sufficiently nice and simple until a skeleton falls out of the closet:
+sometimes we want to clean the connections en masse before they expire.
+
+There few potential scenarios:
+1) removal of an ACL from the interface
+2) removal of an interface
+3) manual action of an operator (in the future).
+
+In order to tackle this, we need to modify the logic which decides whether to requeue the
+connection on the end of the list, or to delete it due to idle timeout:
+
+We define a point in time, and have each worker thread fast-forward through its FIFO,
+in the process looking for sessions that satisfy the criteria, and either keeping them or requeueing them.
+
+To keep the ease of appearance to the outside world, we still process this as an event
+within the connection cleaner thread, but this event handler does as follows:
+1) it creates the bitmap of the sw_if_index values requested to be cleared
+2) for each worker, it waits to ensure there is no cleanup operation in progress (and if there is one,
+it waits), and then makes a copy of the bitmap, sets the per-worker flag of a cleanup operation, and sends an interrupt.
+3) wait until all cleanup operations have completed.
+
+Within the worker interrupt node, we check if the "cleanup in progress" is set,
+and if it is, we check the "fast forward time" value. If unset, we initialize it to value now, and compare the
+requested bitmap of sw_if_index values (pending_clear_sw_if_index_bitmap) with the bitmap of sw_if_index that this worker deals with.
+
+(we set the bit in the bitmap every time we enqueue the packet onto a FIFO - serviced_sw_if_index_bitmap in acl_fa_conn_list_add_session).
+
+If the result of this AND operation is zero - then we can clear the flag of cleanup in progress and return.
+Else we kick off the quantum of cleanup, and make sure we get another interrupt ASAP if that cleanup operation returns non-zero,
+meaning there is more work to do.
+When that operation returns zero, everything has been processed, we can clear the "cleanup-in-progress" flag, and
+zeroize the bitmap of sw_if_index-es requested to be cleaned.
+
+The interrupt node signals its wish to receive an interrupt ASAP by setting interrupt_is_needed
+flag within the per-worker structure. The main thread, while waiting for the
+cleanup operation to complete, checks if there is a request for interrupt,
+and if there is - it sends one.
+
+This approach gives us a way to mass-clean the connections which is reusing the code of the regular idle
+connection cleanup.
+
+One potential inefficiency is the bitmap values set by the session insertion
+in the data path - there is nothing to clear them.
+
+So, if one rearranges the interface placement with the workers, then the cleanups will cause some unnecessary work.
+For now, we consider it an acceptable limitation. It can be resolved by having another per-worker bitmap, which, when set,
+would trigger the cleanup of the bits in the serviced_sw_if_index_bitmap).
+
+=== the end ===
+