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KVM.md

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KVM

KVM is virtualization tool for Linux, built on top of Qemu. It uses hardware virtualiztion (Intel VT, AMD-V).

Note: This document describes how to run kvm "manually". For production environment it is better to use libvirt.

CPU support

How can I tell if I have Intel VT (Virtualization Technology) or AMD-V?

egrep '^flags.*(vmx|svm)' /proc/cpuinfo

If something shows up, your CPU has hardware support for virtualization.

Prepare disk

You can just create some big file (dd if=/dev/zero of=disk.img bs=1M count=10240). Another option is qcow2 file format:

qemu-img create -f qcow2 disk.img 10G

Run

It's simple:

kvm -m 2048 -cdrom some.iso -hda disk.img

Parameter -m 2048 means 2 GB of RAM.

It's not necessary to run kvm under sudo, but you have to be able to use device /dev/kvm - its group should be kvm, so just add yourself to group kvm.

How to release mouse: press both Ctrl Alt :)

Faster disk writes

Only for non-important data.

kvm -drive file=disk.img,cache=unsafe

Networking

By default the virtual machine is connected to some user mode internal network. It's easy to use (no setup needed), VM can reach outside network, but nobody can connect to the VM.

My favorite setup is public virtual bridge - VM connects to the same network as your computer is connected and acts like another computer there. (One problem: it doesn't work with most wireless drivers.)

sudo apt install iproute2 bridge-utils

Setup br0 in /etc/network/interfaces (this is for Debian):

# Replace old eth0 config with br0
auto eth0 br0
# Use old eth0 config for br0, plus bridge stuff
iface br0 inet dhcp
    bridge_ports    eth0
    bridge_stp      off
    bridge_maxwait  0
    bridge_fd       0

Run /etc/init.d/networking restart. Of course do not do this over SSH without physical access to the computer so you are not going to be locked out if anything goes wrong.

Add tap0:

sudo ip tuntap add dev tap0 mode tap user $USER
sudo ip link set dev tap0 up
sudo brctl addif br0 tap0

Alternative: instead of adding tap0 to br0 manually, it can be included in /etc/network/interfaces too. See Debian QEMU wiki.

Run kvm:

kvm -file ...etc. \
    -net nic,macaddr=de:ad:be:ef:11:22 \
    -net tap,ifname=tap0,script=no,downscript=no

If you run multiple kvm instances at once, be sure to assign a different macaddr to each one.

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