Add packet recirculation

Recirculation is a technique to allow a frame to re-enter
frame processing. This is intended to be used after actions
have been applied to the frame with modify the frame in
some way that makes it possible for richer processing to occur.

An example is and indeed targeted use case is MPLS. If an MPLS frame has an
mpls_pop action applied with the IPv4 ethernet type then it becomes
possible to decode the IPv4 portion of the frame. This may be used to
construct a facet that modifies the IPv4 portion of the frame. This is not
possible prior to the mpls_pop action as the contents of the frame after
the MPLS stack is not known to be IPv4.

Design:
* New recirculation action.

  ovs-vswitchd adds a recirculation action to the end of a list of
  datapath actions for a flow when the actions are truncated because
  insufficient flow match information is available to add the next
  OpenFlow action.  The recirculation action is preceded by an action
  to set the skb_mark to an id which can be used to scope a facet lookup
  of a recirculated packet.

  e.g.  pop_mpls(0x0800),dec_ttl becomes pop_mpls(0x800),set(skb_mark(id)),recirculate

* Datapath behaviour

  Then the datapath encounters a recirculate action it:
  + Recalculates the flow key based on the packet
    which will typically have been modified by previous actions
  + As the recirculate action is preceded by a set(skb_mark(id)) action,
    the new match key will now include skb_mark=id.
  + Performs a lookup using the new match key
  + Processes the packet if a facet matches the key or;
  + Makes an upcall if necessary

* No facet behaviour

  + Loop:
    1) translate actions
    2) If there is a recirculate action, execute packet
       and go back to 1) for remaining actions.

Signed-off-by: Simon Horman <horms@verge.net.au>

---

This patch depends on "[PATCH v2.24] datapath: Add basic MPLS support to kernel".

TODO:

* Conclude discussion with Jesse Gross on weather or not packets
  are not output by do_execute_actions() if an output action precedes
  a recirculate action.
* Review of fproto/ofproto-dpif.c by Jesse Gross (and Ben Pfaff?)

Change Log:

v6
* As suggested by Jesse Gross
  - Enforce MAX_RECIRCULATION_DEPTH rather than MAX_RECIRCULATION_DEPTH + 1
    by pre-decrementing rather than post-decrementing loop counter.
  - Stop processing actions when a recirculation action is met
    in validate_and_copy_actions__(). This reflects how
    actions are actually executed and will invalidate actions
    if anything follows recirculation.
  - Add a rate-limited warning for packets dropped because they
    are recirculated too much in ovs_dp_process_received_packet().
  - Pass skb_mark to dp_netdev_sample() rather than using a
    private one.
  - Handle skb_priority when executing recirculation without
    facets in ovs-vswitchd
  - Handle tunnel key when executing recirculation without
    facets in ovs-vswitchd
  - Do not duplicate counting of flow statistics for recirculated packets.
* Merge with patch "Allow recirculation without facets"
* Merge with patch "Avoid recirculation id collision"

v5
* Correct declaration of facet_find_by_id to match definition:
  ovs_be32 -> uint32_t.

* Enhancements to recirculation id code:
  - Allow efficient lookup of facets by their recirculation id
  - Add RECIRCULATION_ID_DUMMY which may be used in cases
    where no facet it used. It is an arbitrary valid id.
  - Also add recirculated element to action_xlate_ctx()
    to use to detect if a recirculation action was added during
    translation. The previous scheme of checking if recirculation_id
    was not RECIRCULATION_ID_NONE is broken for cases where
    the context is initialised with a recirculation_id other than
    RECIRCULATION_ID_NONE. E.g. when RECIRCULATION_ID_DUMMY is used.

* Avoid recirculation id collision

  Avoid recirculation id collision by checking that an id is
  not already associated with a facet.

  Consecutive recirculation ids are used and thus it is possible for
  there to be situations where a very large number of ids have to
  be checked before finding one that is not already associated with a facet.

  To mitigate the performance impact of such situations a limit on
  the number of checks is in place and if no unused recirculation id
  can be found then the miss is handled without facets as this can
  be done using a dummy recirculation id.

rfc4
* Minor enhancement to recirculation id management: Add RECIRCULATE_ID_NONE
  to use instead of using 0 directly.

* Correct calculation of facet->recirculation_ofpacts and
  facet->recirculation_ofpacts_len in subfacet_make_actions()
  in the case of more than one level of recirculation.

* Allow recirculation without facets

  This covers the following cases:
  - Handle flow miss without facet
    + Previously the use of facets was forced if there was
      any chance of a recirculation action. That is, for
      all flows misses of MPLS packets.
  - Packet Out

rfc3

* Use IS_ERR_OR_NULL()

* Handle facet consistency checking by constructing a chain of facets
  from the given facet, to its recirculation parent and then its parent
  until the topmost facet.  If there is no recirculation  the chain will
  be of length one. If there is one recirculation action then the chain
  will be of length two. And so on.

  The topmost facet in the chain can is used to lookup the rule to be
  verified. The chain is then walked from top to bottom, translating
  actions up to the end or the first recirculation action that is
  encountered, whichever comes first. As the code walks down the chain
  it updates the actions that are executed to start of the actions to
  be executed to be just after the end of the actions executed in the
  previous facet in the chain. This is similar to the way that facets
  are created when a recirculation action is encountered.

rfc2
* As suggested by Jesse Gross
  - Update for changes to ovs_dp_process_received_packet()
    to no longer check if OVS_CB(skb)->flow is pre-initialised.
  - Do not add spurious printk debugging to ovs_execute_actions()
  - Do not add spurious debugging messages to commit_set_nw_action()
  - Correct typo in comment above commit_odp_actions().
  - Do not execute recirculation in ovs-vswitchd, rather allow
    the datapath to make an upcall when a recirculation action
    is encountered on execute.
    + This implicitly breaks support for recirculation without facets,
      so for now force all misses of MPLS frames to be handled with
      a facet; and treat handling of recirculation for packet_out as
      a todo item.
  - Use skb_mark for recirculation_id in match. This avoids
    both expanding the match and including a recirculation_id parameter
    with the recirculation action: set_skb_mark should be used before
    the recirculation action.
  - Tidy up ownership of skb in ovs_execute_actions

rfc1
* Initial post