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| vhost-net: a kernel-level virtio-net server
| | #REDIRECT [[UsingVhost]] |
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| == What is vhost-net ==
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| vhost is a kernel-level backend for virtio.
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| The main motivation for vhost is to reduce virtualization
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| overhead for virtio by removing system calls on data path,
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| without guest changes. For virtio-net, this removes up to
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| 4 system calls per packet: vm exit for kick, reentry for kick,
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| iothread wakeup for packet, interrupt injection for packet.
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| vhost is as minimal as possible. It relies on userspace for
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| all setup work.
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| === Status ===
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| vhost is fully functional, and it already shows
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| improvement over userspace virtio.
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| == How to use ==
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| Download, build an install kernel from:
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| kernel:
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| git://git.kernel.org/pub/scm/linux/kernel/git/mst/vhost.git vhost
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| userspace:
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| git://git.kernel.org/pub/scm/linux/kernel/git/mst/qemu-kvm.git vhost
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| === Usage instructions: ===
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| vhost currently requires MSI-X support in guest virtio.
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| This means guests kernel version should be >= 2.6.31.
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| To enable vhost, simply add ",vhost" flag to nic options.
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| Example with tap backend:
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| qemu-system-x86_64 -m 1G disk-c.qcow2 \
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| -net nic,model=virtio,netdev=foo \
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| -netdev type=tap,id=foo,ifname=msttap0,script=/home/mst/ifup,downscript=no,'''vhost=on'''
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| Older (demo) version usage:
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| Example with tap backend:
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| qemu-system-x86_64 -m 1G disk-c.qcow2 \
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| -net tap,ifname=msttap0,script=/home/mst/ifup,downscript=no \
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| -net nic,model=virtio,'''vhost'''
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| Example with raw socket backend:
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| ifconfig eth3 promisc
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| qemu-system-x86_64 -m 1G disk-c.qcow2 \
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| -net raw,ifname=eth3 \
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| -net nic,model=virtio,'''vhost'''
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| Note: in raw socket mode, when binding to a physical
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| ethernet device, host to guest communication
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| will only work if your device is connected to a bridge
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| configured to mirror outgoing packets back at the originating link.
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| If you do not know whether this is the case, this most likely
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| means it isn't. Use another box to access the guest, or use tap.
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| == Limitations ==
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| * vhost currently requires MSI-X support in guest virtio. This means guests kernel version should be >= 2.6.31.
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| * 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.
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| * driver unloading in guest and device hot-unplug are broken, because the relevant code in qemu is stubbed out. Need to implement them.
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| == Performance ==
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| Still tuning performance, especially guest to host.
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| External to system numbers with bridge+tap and 10GE vxge card:
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| qemu with bridge+tap, run with: -cpu host,-rdtscp,+x2apic,
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| host+guest 2.6.33-rc2. mtu 1500.
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| * netperf TCP_STREAM, default setup, 100 secs run
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| native: 81XX Mb/s
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| without vhost-net: 72XX Mb/s
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| with vhost-net: 78XX Mb/s
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| * TCP_RR, 100 secs run
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| native: 48 usec/Trans
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| without vhost-net: 395 usec/Trans
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| with vhost-net: 86 usec/Trans
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| Here are some local numbers coutesy of Shirley Ma:
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| * netperf TCP_STREAM, default setup, 60 secs run
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| guest->host increases from 3XXXMb/s to 5XXXMb/s
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| host->guest increases from 3XXXMb/s to 4XXXMb/s
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| * TCP_RR, 60 secs run
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| guest->host trans/s increases from 2XXX/s to 13XXX/s
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| host->guest trans/s increases from 2XXX/s to 13XXX/s
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| == TODOs ==
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| === vhost-net driver projects ===
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| * profiling would be very helpful, I have not done any yet.
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| * merged buffers.
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| * scalability tuning: figure out the best threading model to use.
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| === qemu projects ===
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| * migration support
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| * level triggered interrupts
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| * driver unloading/hotplug
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| * general cleanup and upstreaming
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| * 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)
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| === virtio projects ===
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| * improve small packet/large buffer performance: support "reposting" buffers, pool for indirect buffers
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| * guest kernel 2.6.31 seems to work well. Under certain workloads,
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| virtio performance has regressed with guest kernels 2.6.32 and up
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| (but still better than userspace). A patch has been posted:
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| http://www.spinics.net/lists/netdev/msg115292.html
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| === projects involing other kernel components and/or networking stack ===
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| * rx mac filtering in tun
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| * 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
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| * improve locking: e.g. RX/TX poll should not need a lock
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| * multicast ICMPs snooping in bridge
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| === long term projects ===
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| * kvm eventfd support for injecting level interrupts
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| * multiqueue (involves all of vhost, qemu, virtio, networking stack)
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| * zero copy tx for tun/raw sockets
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| === Other ===
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| * More testing is always good
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| == Short term plans for MST ==
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| * get vhost net merged in linux kernel 2.6.33
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| * address most vhost qemu TODOs
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| * get vhost support merged in upstream qemu
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| == Short term plans for IBM(Sridhar Samudrala, David Stevens, Shirley MA) ==
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| * Add GSO/checksum offload support to AF_PACKET(raw) sockets.
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| * Mergeable RX buffers support in vhost-net.
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| * Defer SKB allocation in virtio_net receive path.
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