Is fallback vhost_net to qemu for live migrate available？

[Qin Chuanyu <qinchuanyu@huawei.com>]

Hi all

I am participating in a project which try to port vhost_net on Xen。

By change the memory copy and notify mechanism ，currently virtio-net 
with vhost_net could run on Xen with good performance。TCP receive 
throughput of single vnic from 2.77Gbps up to 6Gps。In VM receive 
side，I instead grant_copy with grant_map + memcopy，it efficiently 
reduce the cost of grant_table spin_lock of dom0，So the hole server TCP 
performance from 5.33Gps up to 9.5Gps。

Now I am consider the live migrate of vhost_net on Xen，vhost_net use 
vhost_log for live migrate on Kvm，but qemu on Xen havn't manage the 
hole memory of VM，So I am trying to fallback datapath from vhost_net to 
qemu when doing live migrate ，and fallback datapath from qemu to
vhost_net again after vm migrate to new server。

My question is：
	why didn't vhost_net do the same fallback operation for live migrate on 
KVM，but use vhost_log to mark the dirty page？
	Is there any mechanism fault for the idea of fallback datapath from 
vhost_net to qemu for live migrate？

any question about the detail of vhost_net on Xen is welcome。

Thanks