kvm and ubuntu: High cpu load with network io

Hi,

I have installed ubuntu-server 16.04.3 as host and kvm guest OS on Compulab uSVR with SSD disks. I set up the guest using virt-manager and changed NIC device model to virtio. On the host side the system uses a bridge device. Everything works, but when I receive data from network to the guest, qemu-system-x86_64 uses 100% or more cpu on the host side. My network is currently only 100 Mbps, so the cpu load is way too high. The same happens with NFS and scp. Outbound traffic causes about 70% cpu load.

I tried the same with xen (the same guest running off the same disk as with kvm) and the cpu load was only 16% (same network and getting the same 11.5 MBps to the guest). With xen, the system power consumption was 22 W during the file transfer - with kvm, it was 32 W.

I have tried googling and checking everything I know of, but I can't get the cpu load down with kvm. When the host and guest are idle, the cpu load is close to zero with both kvm and xen.

(On the other hand, disk io is much faster with kvm, so it makes harder to switch to xen..)

Any ideas? Other than switching permanently to xen.. :-)