vhost disables kvm acceleration

vhost disables kvm acceleration

[Asdo]

Hello
I have set up a server with kernel 2.6.37, qemu-kvm-0.13.0 compiled from
source, and libvirt 0.8.3 patched so to enable use of netdev (and hence
vhost).

When I modprobe vhost_net and restart a VM having virtio networking, the
VM crawls at 1/100th of its normal speed.
It seems to me it's about the speed of emulation without kvm
acceleration (I tried that).
If I stop the machine, remove vhost_net module, and restart the VM, it
is normal speed again.

These are the invocations by libvirt:
(note that libvirt autodetects presence of vhost and uses it, the config
of the VM hasn't changed ; also note that they both specify -enable-kvm ...)

without vhost_net module (=fast)

LC_ALL=C
PATH=/usr/local/sbin:/usr/local/bin:/usr/bin:/usr/sbin:/sbin:/bin
QEMU_AUDIO_DRV=none /usr/local/kvm/bin/qemu-system-x86_64 -S -M pc-0.13
-enable-kvm -m 4096 -smp 2,sockets=2,cores=1,threads=1 -name
uarray_server -uuid 7db77cca-addd-4cf4-f7cd-5399d217543e -nodefconfig
-nodefaults -chardev
socket,id=monitor,path=/var/lib/libvirt/qemu/uarray_server.monitor,server,nowait
-mon chardev=monitor,mode=readline -rtc base=utc -boot c -drive
file=/virtualmachines/myserver.raw,if=none,id=drive-virtio-disk0,boot=on,format=raw
-device
virtio-blk-pci,bus=pci.0,addr=0x4,drive=drive-virtio-disk0,id=virtio-disk0
-netdev tap,fd=54,id=hostnet0 -device
virtio-net-pci,netdev=hostnet0,id=net0,mac=52:54:69:94:91:65,bus=pci.0,addr=0x3
-usb -vnc 127.0.0.1:3 -vga cirrus -device
virtio-balloon-pci,id=balloon0,bus=pci.0,addr=0x5


with vhost_net module (=slow)

LC_ALL=C
PATH=/usr/local/sbin:/usr/local/bin:/usr/bin:/usr/sbin:/sbin:/bin
QEMU_AUDIO_DRV=none /usr/local/kvm/bin/qemu-system-x86_64 -S -M pc-0.13
-enable-kvm -m 4096 -smp 2,sockets=2,cores=1,threads=1 -name
uarray_server -uuid 7db77cca-addd-4cf4-f7cd-5399d217543e -nodefconfig
-nodefaults -chardev
socket,id=monitor,path=/var/lib/libvirt/qemu/uarray_server.monitor,server,nowait
-mon chardev=monitor,mode=readline -rtc base=utc -boot c -drive
file=/virtualmachines/myserver.raw,if=none,id=drive-virtio-disk0,boot=on,format=raw
-device
virtio-blk-pci,bus=pci.0,addr=0x4,drive=drive-virtio-disk0,id=virtio-disk0
-netdev tap,fd=52,id=hostnet0,vhost=on,vhostfd=54 -device
virtio-net-pci,netdev=hostnet0,id=net0,mac=52:54:69:94:91:65,bus=pci.0,addr=0x3
-usb -vnc 127.0.0.1:3 -vga cirrus -device
virtio-balloon-pci,id=balloon0,bus=pci.0,addr=0x5


What's the problem?

Thank you

Re: vhost disables kvm acceleration

[Asdo]

On 02/10/2011 07:49 PM, Asdo wrote:
> ... qemu-kvm-0.13.0 compiled from source...
>
> When I modprobe vhost_net and restart a VM having virtio networking, the
> VM crawls at 1/100th of its normal speed.
> [CUT]

I just tried with the newest qemu-kvm 0.14.0-rc1, and exactly the same
problem happens.

Anyone has an idea?

Thank you