Issues around packet capture when secondary process is doing rx/tx

[Stephen Hemminger <stephen@networkplumber.org>]

I have been looking at a problem reported by Sandesh 
where packet capture does not work if rx/tx burst is done in secondary process.

The root cause is that existing rx/tx callback model just doesn't work
unless the process doing the rx/tx burst calls is the same one that
registered the callbacks.

An example sequence would be:
	1. dumpcap (or pdump) as secondary tells pdump in primary to register callback
	2. secondary process calls rx_burst.
	3. rx_burst sees the callback but it has pointer pdump_rx which is not necessarily
	   at same location in primary and secondary process. 
	4. indirect function call in secondary to bad location likely causes crash.

Some possible workarounds.
	1. Keep callback list per-process: messy, but won't crash. Capture won't work
           without other changes. In this primary would register callback, but secondaries
           would not use them in rx/tx burst.

	2. Replace use of rx/tx callback in pdump with change to rte_ethdev to have
           a capture flag. (i.e. don't use indirection).  Likely ABI problems.
           Basically, ignore the rx/tx callback mechanism. This is my preferred
	   solution.

	3. Some fix up mechanism (in EAL mp support?) to have each process fixup
           its callback mechanism.

	4. Do something in pdump_init to register the callback in same process context
	   (probably need callbacks to be per-process). Would mean callback is always
           on independent of capture being enabled.

        5. Get rid of indirect function call pointer, and replace it by index into
           a static table of callback functions. Every process would have same code
           (in this case pdump_rx) but at different address.  Requires all callbacks
           to be statically defined at build time.

The existing rx/tx callback is not safe id rx/tx burst is called from different process
than where callback is registered.