Merge branch 'vhost_net-Avoid-vq-kicks-during-busyloop'

Toshiaki Makita says:

====================
vhost_net: Avoid vq kicks during busyloop

Under heavy load vhost tx busypoll tend not to suppress vq kicks, which
causes poor guest tx performance. The detailed scenario is described in
commitlog of patch 2.
Rx seems not to have that serious problem, but for consistency I made a
similar change on rx to avoid rx wakeups (patch 3).
Additionary patch 4 is to avoid rx kicks under heavy load during
busypoll.

Tx performance is greatly improved by this change. I don't see notable
performance change on rx with this series though.

Performance numbers (tx):

- Bulk transfer from guest to external physical server.
    [Guest]->vhost_net->tap--(XDP_REDIRECT)-->i40e --(wire)--> [Server]
- Set 10us busypoll.
- Guest disables checksum and TSO because of host XDP.
- Measured single flow Mbps by netperf, and kicks by perf kvm stat
  (EPT_MISCONFIG event).

                            Before              After
                          Mbps  kicks/s      Mbps  kicks/s
UDP_STREAM 1472byte              247758                 27
                Send   3645.37            6958.10
                Recv   3588.56            6958.10
              1byte                9865                 37
                Send      4.34               5.43
                Recv      4.17               5.26
TCP_STREAM             8801.03    45794   9592.77     2884

v2:
- Split patches into 3 parts (renaming variables, tx-kick fix, rx-wakeup
  fix).
- Avoid rx-kicks too (patch 4).
- Don't memorize endtime as it is not needed for now.
====================

Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>