Setting up network with QEMU/KVM

alexbrui · 12-04-2015, 01:56 AM

I managed to create VM with clean QEMU/KVM and now try to understand and learn how to setup networking in the most efficient way. I need to access internet from VM and want to get maximum performance with VM network.

From ArchWiki I read that easiest way to get networking is to use "user-mode networking" but performance in this case is low. The most efficient way is to use "tap networking" and enabling vhost. But I can not figure out how to enable this.

Right now I use following code to start VM

Code:

#!/usr/bin/env bash

SPICE_HOST="127.0.0.1"
SPICE_PORT="5930"
VM_NAME="Windows 7"
CPU_COUNT="2"
RAM_COUNT="2048"
DRIVE="Windows7.qcow2"

export QEMU_AUDIO_DRV=alsa

qemu-system-x86_64 \
    -name "$VM_NAME" \
    -cpu host \
    -enable-kvm \
    -smp $CPU_COUNT \
    -m $RAM_COUNT \
    -acpitable file=slic.bin \
    -drive file="$DRIVE",media=disk,if=virtio \
    -vga qxl \
    -spice port=$SPICE_PORT,disable-ticketing \
    -device virtio-serial-pci \
    -device virtserialport,chardev=spicechannel0,name=com.redhat.spice.0 \
    -chardev spicevmc,id=spicechannel0,name=vdagent \
    -rtc base=localtime \
    -soundhw ac97 \
    -usb \
    -usbdevice tablet \
    -device virtio-serial \
    -chardev spicevmc,id=vdagent,name=vdagent \
    -device virtserialport,chardev=vdagent,name=com.redhat.spice.0 \
    -chardev spicevmc,name=usbredir,id=usbredirchardev1 \
    -device usb-redir,chardev=usbredirchardev1,id=usbredirdev1,debug=3 \
    -chardev spicevmc,name=usbredir,id=usbredirchardev2 \
    -device usb-redir,chardev=usbredirchardev2,id=usbredirdev2,debug=3 \
    -chardev spicevmc,name=usbredir,id=usbredirchardev3 \
    -device usb-redir,chardev=usbredirchardev3,id=usbredirdev3,debug=3

I tried to look at the command generated by virt-manager and found this lines related to networking

Code:

-netdev tap,fd=25,id=hostnet0,vhost=on,vhostfd=26
-device virtio-net-pci,netdev=hostnet0,id=net0,mac=52:54:00:b4:2c:53,bus=pci.0,addr=0x3

But adding these lines directly to my script produces error

Code:

TUNGETIFF ioctl() failed: Bad file descriptor
TUNSETOFFLOAD ioctl() failed: Bad file descriptor
qemu-system-x86_64: -netdev tap,fd=25,id=hostnet0,vhost=on,vhostfd=26: vhost-net requested but could not be initialized
qemu-system-x86_64: -netdev tap,fd=25,id=hostnet0,vhost=on,vhostfd=26: Device 'tap' could not be initialized

I suppose this is becase "tap" device is not created and corresponding file handle is not available.

How I can create such device and setup network? Maybe there is another, more convenient way to get working network with maximum performance?

If this is essential, I'm on Debian 8 with kernel 4.2 from backports.

berndbausch · 12-04-2015, 06:58 PM

It would be easier to use libvirt, see libvirt.org. It sets up the taps automatically.
Many cookbooks to set up taps manually exist. I stumbled on this one, slightly old but valid: http://backreference.org/2010/03/26/...face-tutorial/

gradinaruvasile · 12-06-2015, 01:36 PM

And on top of libvirt you migth use virt-manager if you want a gui setup like virtualbox (doesnt need to be installed on the actual host, libvirt can be remotely controlled via ssh).

Also there is Proxmox if you want a complete qemu-based solution with a nice gui, clustering and whatnot.

alexbrui · 12-07-2015, 03:56 AM

Thanks for your replies, guys. I know about libvirt and virt-manager, but I want to learn and understand how to setup network with "pure" qemu/kvm.

gradinaruvasile · 12-07-2015, 04:18 AM

Libvirt launches qemu with the data from the xml files so you can look at the command line for information.