From: Jason Wang <jasowang@redhat.com>
Date: Wed, 25 Nov 2015 15:11:26 +0800

This series tries to add basic busy polling for vhost net. The idea is
simple: at the end of tx/rx processing, busy polling for new tx added
descriptor and rx receive socket for a while. The maximum number of
time (in us) could be spent on busy polling was specified ioctl.

Test A were done through:

- 50 us as busy loop timeout
- Netperf 2.6
- Two machines with back to back connected ixgbe
- Guest with 1 vcpu and 1 queue

Results:
- For stream workload, ioexits were reduced dramatically in medium
  size (1024-2048) of tx (at most -43%) and almost all rx (at most
  -84%) as a result of polling. This compensate for the possible
  wasted cpu cycles more or less. That porbably why we can still see
  some increasing in the normalized throughput in some cases.
- Throughput of tx were increased (at most 50%) expect for the huge
  write (16384). And we can send more packets in the case (+tpkts were
  increased).
- Very minor rx regression in some cases.
- Improvemnt on TCP_RR (at most 17%).

Michael are you going to take this?  It's touching vhost core as
much as it is the vhost_net driver.