As of September 2010, vhost is not included in any released tarballs, so you need the git version.
Vhost provides better latency (10% less than e1000 on my system) and greater throughput (8x the normal virtio, around 7~8 Gigabytes/sec here) for network.
Obtaining the code
- git clone git://git.kernel.org/pub/scm/virt/kvm/qemu-kvm.git
- git checkout refs/remotes/origin/stable-0.13
- cd kvm
- compile as usual
On the host you need a kernel with CONFIG_VHOST_NET=y and in the guest you need a kernel with CONFIG_PCI_MSI=y
example command line
To start a guest with vhost"
- /usr/bin/kvm -m 1024 -netdev type=tap,id=guest0,script=kvm-ifup,vhost=on -device virtio-net-pci,netdev=guest0,mac=00:16:3e:00:00:01 -drive file=/dev/volume01/guest0,if=virtio,boot=on -daemonize