Zero copy is disabled by default. To enable it, set the 'dq-zero-copy'
option to 'true' when configuring the Interface:

ovs-vsctl set Interface dpdkvhostuserclient0
options:vhost-server-path=/tmp/dpdkvhostuserclient0
options:dq-zero-copy=true

When packets from a vHost device with zero copy enabled are destined for
a single 'dpdk' port, the number of tx descriptors on that 'dpdk' port
must be set to a smaller value. 128 is recommended. This can be achieved
like so:

ovs-vsctl set Interface dpdkport options:n_txq_desc=128

Note: The sum of the tx descriptors of all 'dpdk' ports the VM will send
to should not exceed 128. Due to this requirement, the feature is
considered 'experimental'.

Testing of the patch showed a ~8% improvement when switching 512B
packets between vHost devices on different VMs on the same host when
zero copy was enabled on the transmitting device.

Signed-off-by: Ciara Loftus <ciara.loftus@intel.com>
Acked-by: Ilya Maximets <i.maximets@samsung.com>
Signed-off-by: Ian Stokes <ian.stokes@intel.com>