macvtap/vhost TX zero copy support

This patchset add supports for TX zero-copy between guest and host
kernel through vhost. It significantly reduces CPU utilization on the
local host on which the guest is located (It reduced 30-50% CPU usage
for vhost thread for single stream test). The patchset is based on
previous submission and comments from the community regarding when/how
to handle guest kernel buffers to be released. This is the simplest
approach I can think of after comparing with several other solutions.

This patchset includes:

1/8: Add a new sock zero-copy flag, SOCK_ZEROCOPY;

2/8: Add a new device flag, NETIF_F_ZEROCOPY for lower level device
support zero-copy;

3/8: Add a new struct skb_ubuf_info in skb_share_info for userspace
buffers release callback when lower device DMA has done for that skb;

4/8: Add vhost zero-copy callback in vhost when skb last refcnt is gone;
add vhost_zerocopy_add_used_and_signal to notify guest to release TX skb
buffers.

5/8: Add macvtap zero-copy in lower device when sending packet is
greater than 128 bytes.

6/8: Add Chelsio 10Gb NIC to zero copy feature flag

7/8: Add Intel 10Gb NIC zero copy feature flag

8/8: Add Emulex 10Gb NIC zero copy feature flag

The patchset is built against most recent linux 2.6.git. It has passed
netperf/netserver multiple streams stress test on above NICs.

The single stream test results from 2.6.37 kernel on Chelsio:

64K message size: copy_from_user dropped from 40% to 5%; vhost thread
cpu utilization dropped from 76% to 28%

I am collecting more test results against 2.6.39-rc3 kernel and will
provide the test matrix later.

Thanks
Shirley


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