[PATCH V5 0/6 net-next] 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 has integrated V3 review comments from community: 

1. Add more comments on how to use device ZEROCOPY flag;

2. Change device ZEROCOPY to available bit 31

3. Fix skb header linear allocation when virtio_net GSO is not enabled

It has integrated V4 review comments from MST and Sridhar:

1. In vhost, using socket poll wake up for outstanding DMAs

2. Add detailed comments for vhost_zerocopy_signal_used call

3. Add sleep in vhost shutting down instead of busy-wait for outstanding
   DMAs.

4. Copy small packets, don't do zero-copy callback in mavtap, mark it's
   DMA done in vhost

5. change zerocopy to bool in macvtap.

One comment doesn't address:

Should we count the whole page in sk_wmem_alloc for zerocopy buffer? If
so, sock_wfree needs to change too.


This patchset includes:

1/6: Add a new sock zero-copy flag, SOCK_ZEROCOPY;

2/6: Add a new device flag, NETIF_F_ZEROCOPY for lower level device
support zero-copy;

3/6: 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,
which is the last reference count gone;

4/6: Add vhost zero-copy callback in vhost when skb last refcnt is gone;
add vhost_zerocopy_signal_used to notify guest to release TX skb
buffers.

5/6: Add macvtap zero-copy in lower device when sending packet is
greater than 256 bytes to make sure there is enough room for expanding
skb head.

6/6: example: enable Intel 10Gb NIC zero-copy feature flag

The patchset is built against most recent net-next linux 2.6.39-rc7. It
has passed netperf/netserver multiple streams stress test.

Single TCP_STREAM 120 secs test results over ixgbe 10Gb NIC results:

Message BW(Gb/s)qemu-kvm (NumCPU)vhost-net(NumCPU) PerfTop irq/s
4K      7408.57         92.1%           22.6%           1229
4K(Orig)4913.17         118.1%          84.1%           2086    
8K      9129.90         89.3%           23.3%           1141
8K(Orig)7094.55         115.9%          84.7%           2157
16K     9178.81         89.1%           23.3%           1139
16K(Orig)8927.1         118.7%          83.4%           2262
64K     9171.43         88.4%           24.9%           1253
64K(Orig)9085.85        115.9%          82.4%           2229

For message size less or equal than 2K, there is a known KVM guest TX
overrun issue. With this zero-copy patch, the issue becomes more severe,
guest io_exits has tripled than before, so the performance is not good.
Once the TX overrun problem has been addressed, I will retest the small
message size performance.