Allowing space for packet headers in Wintun Tx/Rx

[Jason A. Donenfeld]

Hey guys,

Sorry I'm a bit late to this thread. I'm happy to see there's a
prototype for benchmarking, though I do wonder if this is a bit of
overeager optimization? That is, why is this necessary and does it
actually help?

By returning packets back to the Wintun ring later, more of the ring
winds up being used, which in turn means more cache misses as it spans
additional cache lines. In other words, it seems like this might be
comparing the performance of memcpy+cache no-memcpy+cachemiss. Which
is better, and is it actually measurable? Is it possible that adding
this functionality actually has zero measurable impact on performance?
Given the complexity this adds, it'd be nice to see some numbers to
help make the argument, or perhaps reasoning that's more sophisticated
than my own napkin thoughts here.

Jason