VhostNet

vhost-net: a kernel-level virtio-net server

What is vhost-net

vhost is a kernel-level backend for virtio. The main motivation for vhost is to reduce virtualization overhead for virtio by removing system calls on data path, without guest changes. For virtio-net, this removes up to 4 system calls per packet: vm exit for kick, reentry for kick, iothread wakeup for packet, interrupt injection for packet.

vhost is as minimal as possible. It relies on userspace for all setup work.

Status

vhost is fully functional, and it already shows improvement over userspace virtio.

How to use

Download, build an install kernel from:

kernel:

git://git.kernel.org/pub/scm/linux/kernel/git/mst/vhost.git vhost

userspace:

git://git.kernel.org/pub/scm/linux/kernel/git/mst/qemu-kvm.git vhost

Usage instructions:

vhost currently requires MSI-X support in guest virtio. This means guests kernel version should be >= 2.6.31.

To enable vhost, simply add ",vhost" flag to nic options. Example with tap backend:

qemu-system-x86_64 -m 1G disk-c.qcow2 \
-net tap,ifname=msttap0,script=/home/mst/ifup,downscript=no \
-net nic,model=virtio,vhost

Example with raw socket backend:

ifconfig eth3 promisc
qemu-system-x86_64 -m 1G disk-c.qcow2 \
-net raw,ifname=eth3 \
-net nic,model=virtio,vhost

Note: in raw socket mode, when binding to a physical ethernet device, host to guest communication will only work if your device is connected to a bridge configured to mirror outgoing packets back at the originating link. If you do not know whether this is the case, this most likely means it isn't. Use another box to access the guest, or use tap.

Limitations

vhost currently requires MSI-X support in guest virtio. This means guests kernel version should be >= 2.6.31.
with raw sockets, host to guest, and guest to guest communication on the same host does not always work. Use bridge+tap if you need that.

Performance

Still tuning performance, especially guest to host. Here are some numbers coutesy of Shirley Ma:

netperf TCP_STREAM, default setup, 60 secs run

 guest->host drops from 3XXXMb/s to 1XXXMb/s (regression)
 host->guest increases from 3XXXMb/s to 4XXXXMb/s

TCP_RR, 60 secs run

 guest->host trans/s increases from 2XXX/s to 13XXX/s
 host->guest trans/s increases from 2XXX/s to 13XXX/s

TODOs

Fix guest to host performance regression - working on it now

vhost-net driver projects

profiling would be very helpful, I have not done any yet.
vm exit mitigation for TX - working on it now.
logging support with dirty page tracking in kernel - working on it now
merged buffers.

qemu projects

migration support
level triggered interrupts
general cleanup and upstreaming
upstream support for injecting interrupts from kernel,

from qemu-kvm.git to qemu.git (this is a vhost dependency, without it vhost can't be upstreamed, or it can, but without real benefit)

projects involing other kernel components and/or networking stack

extend raw sockets to support GSO/checksum offloading,

and teach vhost to use that capability [one way to do this: virtio net header support] will allow working with e.g. macvlan

improve locking: e.g. RX/TX poll should not need a lock
kvm eventfd support for injecting level interrupts

long term projects

multiqueue (involves all of vhost, qemu, virtio, networking stack)

Other

More testing is always good