Difference between revisions of "QEMU"

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From the [http://wiki.qemu.org/Main_Page QEMU about page],
From the [http://wiki.qemu.org/Main_Page QEMU about page],

Revision as of 08:56, 13 June 2012

zh-CN:QEMU From the QEMU about page,

QEMU is a generic and open source machine emulator and virtualizer.

When used as a machine emulator, QEMU can run OSes and programs made for one machine (e.g. an ARM board) on a different machine (e.g. your own PC). By using dynamic translation, it achieves very good performance.

When used as a virtualizer, QEMU achieves near native performances by executing the guest code directly on the host CPU. QEMU supports virtualization when executing under the Xen hypervisor or using the KVM kernel module in Linux. When using KVM, QEMU can virtualize x86, server and embedded PowerPC, and S390 guests.


Installing QEMU

Depending on your needs, you can choose to install either qemu or qemu-kvm from the official repositories. qemu includes support for emulating a wide variety of machine architectures, while qemu-kvm only supports virtualizing your host architecture using KVM. It is strongly recommended to use KVM whenever possible; see #Using the Kernel-based Virtual Machine for more information. In the current version of QEMU (>= 0.15.0), you can still use KVM with the qemu package, if supported by your processor and kernel, provided that you start QEMU with the -enable-kvm argument; this was not the case for older versions of QEMU (< 0.15.0), when not all KVM-related functions had been merged into upstream QEMU.

Creating a hard disk image

To run QEMU you will need a hard disk image, unless you are booting a live system from CD-ROM or the network (and not doing so to install an operating system to a hard disk image). A hard disk image is a file which stores the contents of the emulated hard disk.

A hard disk image may simply contain the literal contents, byte for byte, of the hard disk. This is usually called raw format, and it provides the least I/O overhead, although the images may take up a large amount of space.

Alternatively, the hard disk image can be in a format such as qcow2 that can save enormous amounts of space by only allocating space to the image file when the guest operating system actually writes to those sectors on its virtual hard disk. The image appears as the full size to the guest operating system, even though it may take up only a very small amount of space on the host system.

QEMU provides the qemu-img command to create hard disk images. The following command creates a 4GB image named myimage.qcow2 in the qcow2 format:

$ qemu-img create -f qcow2 myimage.qcow2 4G

You may use -f raw to create a raw disk instead, although you can also do so simply by creating a file of the needed size using dd or fallocate.

Preparing the installation media

To install an operating system into your disk image, you need the installation media (e.g. CD-ROM, floppy, or ISO image) for the operating system.

Tip: If you would like to run an Arch Linux virtual machine, you can install it using the official installation media for Arch Linux. It is also possible to set up an Arch Linux virtual machine without the installation media, provided that your host machine is running Arch Linux, although this is more difficult; it is detailed here.

The installation media should not be mounted because QEMU accesses the media directly. Also, if using physical media (e.g. CD-ROM or floppy), it is a good idea to first dump the media to a file because this both improves performance and does not require you to have direct access to the devices (that is, you can run QEMU as a regular user without having to change access permissions on the media's device file). For example, if the CD-ROM device node is named /dev/cdrom, you can dump it to a file with the command:

# dd if=/dev/cdrom of=mycdimg.iso

Do the same for floppies:

# dd if=/dev/fd of=myfloppy.img

Installing the operating system

To install the operating system on the disk image, you must attach both the disk image and the installation media to the virtual machine, and have it boot from the installation media.

This is the first time you will need to start the emulator. By default, QEMU will show the virtual machine's video output in a window. One thing to keep in mind: when you click inside the QEMU window, the mouse pointer is grabbed. To release it press Template:Keypress.

Warning: QEMU should never be run as root. If you must launch it in a script as root, you should use the -runas option to make QEMU drop root privileges.

If you need to use a bootable floppy, run QEMU with:

$ qemu -cdrom [[cdrom''image]] -fda [[floppy''image]] -boot a [[hd_image]]

or if you are on a x86_64 system (will avoid many problems afterwards):

$ qemu-system-x86_64 -cdrom [[cdrom''image]] -fda [[floppy''image]] -boot a [[hd_image]]

If your CD-ROM is bootable or you are using ISO files, run QEMU with:

$ qemu -cdrom [[cdrom''image]] -boot d [[hd''image]]

or if you are on a x86_64 system (will avoid many problems afterwards):

$ qemu-system-x86_64 -cdrom [[cdrom''image]] -boot d [[hd''image]]

Now, using the installer for the OS, partition the virtual hard disk, format the partitions, and install the OS.

Note: See #Windows-specific notes if you are installing Windows in your virtual machine.
Note: If you need to replace floppies or CDs as part of the installation process, you can use the QEMU machine monitor (press Template:Keypress in the virtual machine's window) to remove and attach storage devices to a virtual machine. Type info block to see the block devices, and use the change command to swap out a device. Press Template:Keypress to go back to the virtual machine.

Running the installed system

After the operating system has finished installing, you can boot the disk image by itself, without the installation media.

To run the system simply use a command like:

$ qemu [hd_image]
Note: If possible, you should consider using KVM for improved performance.

Using the Kernel-based Virtual Machine

KVM is a full virtualization solution for Linux on x86 hardware containing virtualization extensions (Intel VT or AMD-V). It consists of a loadable kernel module, kvm.ko, that provides the core virtualization infrastructure and a processor specific module, kvm-intel.ko or kvm-amd.ko. Using KVM, one can run multiple virtual machines running unmodified Linux or Windows images. Each virtual machine has private virtualized hardware: a network card, disk, graphics adapter, etc.

Tip: It is recommended to use KVM whenever possible because it will greatly improve the performance of the running virtual machine compared to QEMU emulating the virtual machine entirely in software.

KVM requires an x86 machine running a recent ( >= 2.6.22) Linux kernel on an Intel processor with VT-x (Virtualization Technology) extensions, or an AMD processor with SVM (Secure Virtual Machine) extensions—AMD-V is the marketing term for AMD's SVM. It is included in the mainline Linux kernel since 2.6.20 and is enabled by default in the Arch Linux kernel.

qemu >= 0.15.0 has full support for KVM, as the qemu-kvm tree has been completely merged into the upstream QEMU tree. Therefore, there should be no difference between qemu -enable-kvm and qemu-kvm if your version of qemu is >= 0.15.0.

Note: QEMU in older versions ( < 0.15.0) does have initial KVM support (qemu -enable-kvm), but it is not recommended to use this, as many KVM-related functions had not been implemented in upstream QEMU. Instead, for older versions of QEMU you should go for the qemu-kvm package in the official repositories, which is released by the KVM development team and contains all of the latest features (and bug-fixes) of KVM userspace. Please refer to the KVM page itself, for more information on using QEMU with KVM on Arch Linux.

To take advantage of KVM, you need a compatible processor (the following command must return something on the screen):

$ grep -E "(vmx|svm)" --color=always /proc/cpuinfo

And load the appropriate module from your /etc/rc.conf.

  • For Intel® processors, add kvm-intel to your MODULES array in /etc/rc.conf
  • for AMD® processors, add kvm-amd to your MODULES array in /etc/rc.conf

Also, you will need to add yourself to the group kvm.

# gpasswd -a <Your_User_Account> kvm

Overlay images

A good idea is to use overlay images. This way you can a create hard disk image once and tell QEMU to store changes in an external file. This makes it easy to revert the virtual machine's disk to a previous state.

To create an overlay image, type:

$ qemu-img create -b [[base''image]] -f qcow2 [[overlay''image]]

After that you can run qemu with:

$ qemu [overlay_image]

or if you are on a x86_64 system:

$ qemu-system-x86_64 [overlay_image]

and the original image will be left untouched. One hitch, the base image cannot be renamed or moved, the overlay remembers the base's full path.

Moving data between host and guest OS


Data can be shared between the host and guest OS using any network protocol that can transfer files, such as NFS, SMB, NBD, HTTP, FTP, or SSH, provided that you have set up the network appropriately and enabled the appropriate services.

The default user-mode networking allows the guest to access the host OS at the IP address Any servers that you are running on your host OS, such as a SSH server or SMB server, will be accessible at this IP address. So on the guests, you can mount directories exported on the host via SMB or NFS, or you can access the host's HTTP server, etc. It will not be possible for the host OS to access servers running on the guest OS, but this can be done with other network configurations (see #Tap Networking with QEMU).

QEMU's built-in SMB server

Note: QEMU's "built-in" SMB server is currently (as of qemu-1.0.1-1) broken because it does not specify the state_directory option in the smb.conf file it writes. This issue is fixed in upstream QEMU.

QEMU's documentation says it has a "built-in" SMB server, but actually it just starts up Samba with an automatically generated configuration file and makes it accessible to the guest at a different IP address ( by default). This only works for user networking, and this isn't necessarily very useful since the guest can also access the normal Samba service on the host if you have set up shares on it.

To enable this feature, start QEMU with a command like:

$ qemu [hd_image] -net nic -net user,smb=/path/to/shared/dir

where /path/to/shared/dir is a directory that you want to share between the guest and host.

Then, in the guest, you will be able to access the shared directory on the host with the share name "qemu". For example, in Windows Explorer you would go to \\\qemu.

Mounting a partition inside a raw disk image

When the virtual machine is not running, it is possible to mount partitions that are inside a raw disk image file by setting them up as loopback devices. This does not work with disk images in special formats, such as qcow2, although those can be mounted using qemu-nbd.

Warning: You must make sure to unmount the partitions before running the virtual machine again. Otherwise data corruption could occur, unless you had mounted the partitions read-only.

With manually specifying byte offset

One way to mount a disk image partition is to mount the disk image at a certain offset using a command like the following:

# mount -o loop,offset=32256 [hd_image] [tmp_dir]

The offset=32256 option is actually passed to the losetup program to set up a loopback device that starts at byte offset 32256 of the file and continues to the end. This loopback device is then mounted. You may also use the sizelimit option to specify the exact size of the partition, but this is usually unnecessary.

Depending on your disk image, the needed partition may not start at offset 32256. Run fdisk -l [hd_image] to see the partitions in the image. fdisk gives the start and end offsets in 512-byte sectors, so multiply by 512 to get the correct offset to pass to {{ic|mount}.

With loop module autodetecting partitions

The Linux loop driver actually supports partitions in loopback devices, but it is disabled by default. To enable it, do the following:

  • Get rid of all your loopback devices (unmount all mounted images, etc.).
  • Unload the loop module.
# modprobe -r loop
  • Load the loop module with the max_part parameter set.
# modprobe loop max_part=15
Tip: You can put an entry in /etc/modprobe.d to load the loop module with max_part=15 every time, or you can put loop.max_part=15 on the kernel command line, depending on whether you have the loop.ko module built into your kernel or not.

Set up your image as a loopback device:

# losetup -f [os_image]

Then, if the device created was /dev/loop0, additional devices /dev/loop0pX will have been automatically created, where X is the number of the partition. These partition loopback devices can be mounted directly. For example:

# mount /dev/loop0p1 [tmp_dir]

With kpartx

kpartx from the multipath-tools package can read a partition table on a device and create a new device for each partition. For example:

# kpartx -a /dev/loop0

Mounting qcow2 image

You may mount a qcow2 image using qemu-nbd. See [Wikibooks].

Using any real partition as the single primary partition of a hard disk image

Sometimes, you may wish to use one of your system partitions from within QEMU. Using a raw partition for a virtual machine will improve performance, as the read and write operations do not go through the filesystem layer on the physical host. Such a partition also provides a way to share data between the host and guest.

In Arch Linux, device files for raw partitions are, by default, owned by root and the disk group. If you would like to have a non-root user be able to read and write to a raw partition, you need to change the owner of the partition's device file to that user.

Warning: Although it is possible, it is not recommended to allow virtual machines to alter critical data on the host system, such as the root partition.
Warning: You must not mount a filesystem on a partition read-write on both the host and the guest at the same time. Otherwise, data corruption will result.

After doing so, you can attach the partition to a QEMU virtual machine as a virtual disk.

However, things are a little more complicated if you want to have the entire virtual machine contained in a partition. In that case, there would be no disk image file to actually boot the virtual machine since you cannot install a bootloader to a partition that is itself formatted as a filesystem and not as a partitioned device with a MBR (FIXME: Isn't that actually possible, since filesystems don't begin on the first sector?). Such a virtual machine can be booted either by specifying the kernel and initrd manually, or by simulating a disk with a MBR by using linear RAID.

By specifying kernel and initrd manually

QEMU supports loading Linux kernels and init ramdisks directly, thereby circumventing bootloaders such as GRUB. It then can be launched with the physical partition containing the root filesystem as the virtual disk, which will not appear to be partitioned. This is done by issuing a command similar to the following:

$ qemu -kernel /boot/vmlinuz-linux -initrd /boot/initramfs-linux.img -append root=/dev/sda /dev/sda3

In the above example, the physical partition being used for the guest's root filesystem is /dev/sda3 on the host, but it shows up as /dev/sda on the guest.

You may, of course, specify any kernel and initrd that you want, and not just the ones that come with Arch Linux.

Simulate virtual disk with MBR using linear RAID

A more complicated way to have a virtual machine use a physical partition, while keeping that partition formatted as a filesystem and not just having the guest partition the partition as if it were a disk, is to simulate a MBR for it so that it can boot using a bootloader such as GRUB.

You can do this using software RAID in linear mode (you need the linear.ko kernel driver) and a loopback device: the trick is to dynamically prepend a master boot record (MBR) to the real partition you wish to embed in a QEMU raw disk image.

Suppose you have a plain, unmounted /dev/hdaN partition with some filesystem on it you wish to make part of a QEMU disk image. First, you create some small file to hold the MBR:

$ dd if=/dev/zero of=/path/to/mbr count=32

Here, a 16 KB (32 * 512 bytes) file is created. It is important not to make it too small (even if the MBR only needs a single 512 bytes block), since the smaller it will be, the smaller the chunk size of the software RAID device will have to be, which could have an impact on performance. Then, you setup a loopback device to the MBR file:

# losetup -f /path/to/mbr

Let's assume the resulting device is /dev/loop0, because we would not already have been using other loopbacks. Next step is to create the "merged" MBR + /dev/hdaN disk image using software RAID:

 # modprobe linear
 # mdadm --build --verbose /dev/md0 --chunk=16 --level=linear --raid-devices=2 /dev/loop0 /dev/hdaN

The resulting /dev/md0 is what you will use as a QEMU raw disk image (do not forget to set the permissions so that the emulator can access it). The last (and somewhat tricky) step is to set the disk configuration (disk geometry and partitions table) so that the primary partition start point in the MBR matches the one of /dev/hdaN inside /dev/md0 (an offset of exactly 16 * 512 = 16384 bytes in this example). Do this using fdisk on the host machine, not in the emulator: the default raw disc detection routine from QEMU often results in non kilobyte-roundable offsets (such as 31.5 KB, as in the previous section) that cannot be managed by the software RAID code. Hence, from the the host:

# fdisk /dev/md0

Press Template:Keypress to enter the expert menu. Set number of 's'ectors per track so that the size of one cylinder matches the size of your MBR file. For two heads and a sector size of 512, the number of sectors per track should be 16, so we get cylinders of size 2x16x512=16k.

Now, press Template:Keypress to return to the main menu.

Press Template:Keypress and check that the cylinder size is now 16k.

Now, create a single primary partition corresponding to /dev/hdaN. It should start at cylinder 2 and end at the end of the disk (note that the number of cylinders now differs from what it was when you entered fdisk.

Finally, 'w'rite the result to the file: you are done. You know have a partition you can mount directly from your host, as well as part of a QEMU disk image:

$ qemu -hdc /dev/md0 [...]

You can of course safely set any bootloader on this disk image using QEMU, provided the original /dev/hdaN partition contains the necessary tools.


User-mode networking

By default, without any -net arguments, QEMU will use user-mode networking with a built-in DHCP server. Your virtual machines will be assigned an IP address when they run their DHCP client, and they will be able to access the physical host's network through IP masquerading done by QEMU. This only works with the TCP and UDP protocols, so ICMP, including ping, will not work.

This default configuration allows your virtual machines to easily access the Internet, provided that the host is connected to it, but the virtual machines will not be directly visible on the external network, nor will virtual machines be able to talk to each other if you start up more than one concurrently.

QEMU's user-mode networking can offer more capabilities such as built-in TFTP or SMB servers, or attaching guests to virtual LANs so that they can talk to each other. See the QEMU documentation on the -net user flag for more details.

However, user-mode networking has limitations in both utility and performance. More advanced network configurations require the use of tap devices or other methods.

Tap networking with QEMU

Basic idea

Tap devices are a Linux kernel feature that allows you to create virtual "tap" network interfaces that appear as real network interfaces. Packets sent to a "tap" interface are delivered to a userspace program, such as QEMU, that has bound itself to the interface.

QEMU can use tap networking for a virtual machine so that packets sent to the tap interface will be sent to the virtual machine and appear as coming from a network interface (usually an Ethernet interface) in the virtual machine. Conversely, everything that the virtual machine sends through its network interface will appear on the tap interface.

Tap devices are supported by the Linux bridge drivers, so it is possible to bridge together tap devices with each other and possibly with other host interfaces such as eth0. This is desirable if you want your virtual machines to be able to talk to each other, or if you want other machines on your LAN to be able to talk to the virtual machines.

Bridge virtual machines to external network

The following describes how to bridge a virtual machine to a host interface such as eth0, which is probably the most common configuration. This configuration makes it appear that the virtual machine is located directly on the external network, on the same Ethernet segment as the physical host machine.

Warning: Beware that since your virtual machines will appear directly on the external network, this may expose them to attack. Depending on what resources your virtual machines have access to, you may need to take all the precautions you normally would take in securing a computer to secure your virtual machines.

We will replace the normal Ethernet adapter with a bridge adapter and bind the normal Ethernet adapter to it. See http://en.gentoo-wiki.com/wiki/KVM#Networking_2 .

1. Make sure that the following packages are installed:

2. Enable IPv4 forwarding by changing net.ipv4.ip_forward = 0 to net.ipv4.ip_forward = 1 in /etc/sysctl.conf.

3. Add bridge and tun to your MODULES array in /etc/rc.conf:

MODULES=( ... bridge tun)

4. Configure your bridge br0 to have your real Ethernet adapter (assuming eth0 for the rest of this guide) in it, in /etc/conf.d/bridges:

 control_br0="setfd br0 0"
Note: This is not described anywhere, but adding the control_br0 line is vital for the bridge to work! For more details look here: FS#16625

5. Change your networking configuration so that you just bring up your real Ethernet adapter without configuring it, allowing real configuration to happen on the bridge interface. In /etc/rc.conf:

 eth0="eth0 up"
 INTERFACES=(eth0 br0)

Remember, especially if you are doing DHCP, it is essential that the bridge comes up AFTER the real adapter, otherwise the bridge will not be able to talk to anything to get a DHCP address!

If you have been giving eth0 a static IP address rather than using DHCP, give br0 similar settings:

br0="br0 netmask broadcast"
INTERFACES=(eth0 br0)
gateway="default gw"

and then in /etc/resolv.conf:

domain lan

6. Install the script that QEMU uses to bring up the tap adapter in /etc/qemu-ifup with root:kvm 750 permissions:

 echo "Executing /etc/qemu-ifup"
 echo "Bringing up $1 for bridged mode..."
 sudo /sbin/ifconfig $1 promisc up
 echo "Adding $1 to br0..."
 sudo /usr/sbin/brctl addif br0 $1
 sleep 2

7. Use visudo to add the following to your sudoers file:

 Cmnd_Alias      QEMU=/sbin/ifconfig,/sbin/modprobe,/usr/sbin/brctl,/usr/bin/tunctl

8. Make sure the user(s) wishing to use this new functionality are in the kvm group. Exit and log in again if necessary.

9. You launch QEMU using the following run-qemu script:

 IFACE=$(sudo tunctl -b -u $USERID)
 printf -v macaddr "52:54:%02x:%02x:%02x:%02x" \
   $(( $RANDOM & 0xff)) $(( $RANDOM & 0xff )) \
   $(( $RANDOM & 0xff)) $(( $RANDOM & 0xff ))
 qemu-kvm -net nic,macaddr=$macaddr -net tap,ifname="$IFACE" $*
 sudo tunctl -d $IFACE &> /dev/null

Then to launch a VM, do something like this

$ run-qemu -hda myvm.img -m 512 -vga std

10. If you cannot get a DHCP address in the host, it might be because iptables are up by default in the bridge. In that case (from http://www.linux-kvm.org/page/Networking ):

 # cd /proc/sys/net/bridge
 # ls
 bridge-nf-call-arptables  bridge-nf-call-iptables
 bridge-nf-call-ip6tables  bridge-nf-filter-vlan-tagged
 # for f in bridge-nf-*; do echo 0 > $f; done

And if you still cannot get networking to work, see: Linux_Containers#Bridge_device_setup

Host-only networking

If the bridge is given an IP address and traffic destined for it is allowed, but no "real" interface (e.g. eth0) is also connected to the bridge, then the virtual machines will be able to talk to each other and the physical host. However, they will not be able to talk to anything on the external network, provided that you do not set up IP masquerading on the physical host. This configuration is called "host-only" networking by other virtualization software such as VirtualBox.

You may want to have a DHCP server running on the bridge interface to service the virtual network. For example, to use the subnet with Dnsmasq as the DHCP server:

# ifconfig br0 up
# dnsmasq --interface=br0 --bind-interfaces --dhcp-range=,

Internal networking

If you do not give the bridge an IP address and add an iptables rule to drop all traffic to the bridge in the INPUT chain, then the virtual machines will be able to talk to each other, but not to the physical host or to the outside network. This configuration is called "internal" networking by other virtualization software such as VirtualBox. You will need to either assign static IP addresses to the virtual machines or run a DHCP server on one of them.

Link-level address caveat

By giving the -net nic argument to QEMU, it will, by default, assign a virtual machine a network interface with the link-level address 52:54:00:12:34:56. However, when using bridged networking with multiple virtual machines, it is essential that each virtual machine has a unique link-level (MAC) address on the virtual machine side of the tap device. Otherwise, the bridge will not work correctly, because it will receive packets from multiple sources that have the same link-level address. This problem occurs even if the tap devices themselves have unique link-level addresses because the source link-level address is not rewritten as packets pass through the tap device.

To solve this problem, the last 8 digits of the link-level address of the virtual NICs should be randomized, as in the script above, to make sure that each virtual machine has a unique link-level address.

Networking with VDE2

What is VDE?

VDE stands for Virtual Distributed Ethernet. It started as an enhancement of uml_switch. It is a toolbox to manage virtual networks.

The idea is to create virtual switches, which are basically sockets, and to "plug" both physical and virtual machines in them. The configuration I show here is quite simple; However, VDE is much more powerful than this, it can plug virtual switches together, run them on different hosts and monitor the traffic in the switches. Your are invited to read the documentation of the project.

The advantage of this method is you do not have to add sudo privileges to your users. Regular users should not be allowed to run modprobe.


VDE is in the official repositories, so...

# pacman -S vde2

In my config, I use tun/tap to create a virtual interface on my host. Load the tun module (or add it to your MODULES array in rc.conf):

# modprobe tun

Now create the virtual switch:

# vde_switch -tap tap0 -daemon -mod 660 -group kvm

This line creates the switch, creates tap0, "plugs" it, and allows the users of the group kvm to use it.

The interface is plugged in but not configured yet. Just do it:

# ifconfig tap0 netmask

That is all! Now, you just have to run KVM with these -net options as a normal user:

$ qemu-kvm -net nic -net vde -hda ...

Configure your guest as you would do in a physical network. I gave them static addresses and let them access the WAN using IP forwarding and masquerading on my host:

# echo "1" > /proc/sys/net/ipv4/ip_forward
# iptables -t nat -A POSTROUTING -s -o eth0 -j MASQUERADE

Putting it together

I added this init script to run all this at start-up:


. /etc/rc.conf
. /etc/rc.d/functions
case "$1" in
    stat_busy "Starting VDE Switch"
    vde_switch -tap tap0 -daemon -mod 660 -pidfile $PIDFILE -group kvm
    if [ $? -gt 0 ]; then
        echo "1" > /proc/sys/net/ipv4/ip_forward &&  \
        iptables -t nat -A POSTROUTING -s -o eth0 -j MASQUERADE &&  \
        ifconfig tap0 netmask && \
        stat_done || stat_fail
    stat_busy "Stopping VDE Switch"
    # err.. well, i should remove the switch here...
    $0 stop
    sleep 1
    # Aem.. As long as stop) is not implemented, this just fails
    $0 start
    echo "usage: $0 {start|stop|restart}"  
exit 0

Well, I know it is dirty and could be more configurable. Feel free to improve it. VDE has an rc script too, but I had to make one anyway for the IP forwarding stuff.

Alternative method

If the above method does not work or you do not want to mess with kernel configs, TUN, dnsmasq and iptables you can do the following for the same result.

# vde_switch -daemon -mod 660 -group kvm
# slirpvde --dhcp --daemon

Then to start the vm with a connection to the network of the host:

$ kvm -net nic,macaddr=52:54:00:00:EE:03 -net vde whatever.qcow

Improving networking performance

The performance of virtual networking should be better with tap devices and bridges than with user-mode networking or vde, since tap devices and bridges are implemented in-kernel.

In addition, networking performance can be improved by assigning virtual machines a virtio network device rather than the default emulation of an e1000 NIC. To do this, add a model=virtio flag to the -net nic option:

-net nic,model=virtio

This will only work if the guest machine has a driver for virtio network devices. Linux does, and the required driver (virtio_net) is included with Arch Linux, but there is no guarantee that virtio networking will work with arbitrary operating systems. There do exist virtio drivers for Windows, but you need to install them manually.


QEMU can use the following different graphic outputs: std, cirrus, vmware, qxl, xenfs and vnc. With the vnc option you can run your guest standalone and connect to it via VNC. Other options are using std, vmware, cirrus:


With -vga std you can get a resolution of up to 2560 x 1600 pixels.


Although it is a bit buggy, it performs better than std and cirrus. On the guest, install the VMware drivers. For Arch Linux guests:

# pacman -S xf86-video-vmware xf86-input-vmmouse


If you do not want to see the graphical output from your virtual machine because you will be accessing it entirely through the network or serial port, you can run QEMU with the -nographic option.

Graphical front-ends for QEMU

Unlike other virtualization progrems such as VirtualBox and VMware, QEMU does not provide a GUI to manage virtual machines (other than the window that appears when running a virtual machine), nor does it provide a way to create persistent virtual machines with saved settings. All parameters to run a virtual machine must be specified on the command line at every launch, unless you have created a custom script to start your virtual machine(s). However, there are several GUI front-ends for QEMU:

Windows-specific notes

Choosing a Windows version

QEMU can run any version of Windows. However, 98, Me and XP will run at quite a low speed. You should choose either Windows 95 or Windows 2000. Surprisingly, 2000 seems to run faster than 98. The fastest one is 95, which can from time to time make you forget that you are running an emulator :)

If you own both Win95 and Win98/WinME, then 98lite (from http://www.litepc.com) might be worth trying. It decouples Internet Explorer from operating system and replaces it with original Windows 95 Explorer. It also enables you to do a minimal Windows installation, without all the bloat you normally cannot disable. This might be the best option, because you get the smallest, fastest and most stable Windows this way.

It is possible to run Windows PE in QEMU.

Windows 95 boot floppy

If you are using the Windows 95 boot floppy, choosing SAMSUNG as the type of CD-ROM seems to work.

Windows 2000 installation bug

There are problems when installing Windows 2000. Windows setup will generate a lot of edb*.log files, one after the other containing nothing but blank spaces in C:\WINNT\SECURITY which quickly fill the virtual hard disk. A workaround is to open a Windows command prompt as early as possible during setup (by pressing Template:Keypress) which will allow you to remove these log files as they appear by typing:

del %windir%\security\*.log
Note: According to the official QEMU website, "Windows 2000 has a bug which gives a disk full problem during its installation. When installing it, use the -win2k-hack QEMU option to enable a specific workaround. After Windows 2000 is installed, you no longer need this option (this option slows down the IDE transfers)."

Optimizing Windows 9X CPU usage

Windows 9X uses an idle loop instead of the HLT (halt) instruction. Consequently, the emulator will consume all CPU resources when running Windows 9X guests -- even if no work is being done. This only applies to DOS and DOS-based Windows versions (3.X, 95/98/ME) -- NT-based and later Windows versions are not affected.

To resolve this issue, install Rain, Waterfall or CpuIdle in the Windows 9X guest. (Rain might be preferred because it does only what is needed -- replacing the idle loop with the HLT instruction -- and nothing more.)

See Tutorial: Windows 95/98 guest OSes for more information.

Remote Desktop Protocol

If you use a MS Windows guest, you might want to use RDP to connect to your guest VM. Use: (if you are using a VLAN or are not in the same network as the guest)

$ qemu -nographic -net user,hostfwd=tcp::5555-:3389

Then connect with either rdesktop or freerdp to the guest, for example:

$ xfreerdp -g 2048x1152 localhost:5555 -z -x lan

Windows virtio drivers

You can use virtio devices with Windows if you install the virtio guest drivers for Windows.

General problems

Keyboard seems broken or the arrow keys do not work

Should you find that some of your keys do not work or "press" the wrong key (in particular, the arrow keys), you likely need to specify your keyboard layout as an option. The keyboard layouts can be found in /usr/share/qemu/keymaps.

qemu -k [keymap] [disk_image]

Virtual machine runs too slowly

There are a number of techniques that you can use to improve the performance if your virtual machine. For example:

  • Use KVM if possible (see #Using the Kernel-based Virtual Machine).
  • Make sure you have assigned the virtual machine enough memory. By default, QEMU only assigns 128MiB of memory to each virtual machine. Use the -m option to assign more memory. For example, -m 1024 runs a virtual machine with 1024MiB of memory.
  • If the host machine has multiple CPUs, assign the guest more CPUs using the -smp option.
  • Use the -cpu host option to make QEMU emulate the host's exact CPU. If you don't do this, it may be trying to emulate a more generic CPU.
  • If supported by drivers in the guest operating system, use virtio for network and/or block devices. For example:
$ qemu -net nic,model=virtio -net tap,if=tap0,script=no -drive file=mydisk.raw,media=disk,if=virtio
  • Use TAP devices instead of user-mode networking.
  • If the guest OS is doing heavy writing to its disk, you may benefit from certain mount options on the host's filesystem. For example, you can mount an ext4 filesystem with the option barrier=0. You should read the documentation for any options that you change, since sometimes performance-enhancing options for filesystems come at the cost of data integrity.
  • If you are running multiple virtual machines concurrently that all have the same operating system installed, you can save memory by enabling kernel same-page merging:
# echo 1 > /sys/kernel/mm/ksm/run
  • In some cases, memory can be reclaimed from running virtual machines by running a memory ballooning driver in the guest operating system and launching QEMU with the -balloon virtio option.

Starting QEMU virtual machines on boot

With libvirt

If a virtual machine is set up with libvirt, it can be configured through the virt-manager GUI to start at host boot by going to the Boot Options for the virtual machine and selecting "Start virtual machine on host boot up".

Custom script

To run QEMU VMs on boot, you can use following rc-script and config.

Config file options
QEMU_MACHINES List of VMs to start
qemu_${vm}_type QEMU binary to call. If specified, will be prepended with /usr/bin/qemu- and that binary will be used to start the VM. I.e. you can boot e.g. qemu-system-arm images with qemu_my_arm_vm_type="system-arm". If not specified, /usr/bin/qemu will be used.
qemu_${vm} QEMU command line to start with. Will always be prepended with -name ${vm} -pidfile /var/run/qemu/${vm}.pid -daemonize -nographic.
qemu_${vm}_haltcmd Command to shutdown VM safely. I am using -monitor telnet:.. and power off my VMs via ACPI by sending system_powerdown to monitor. You can use ssh or some other ways.
qemu_${vm}_haltcmd_wait How much time to wait for safe VM shutdown. Default is 30 seconds. rc-script will kill qemu process after this timeout.

Config file example:

# VMs that should be started on boot
# use the ! prefix to disable starting/stopping a VM

# NOTE: following options will be prepended to qemu_${vm}
# -name ${vm} -pidfile /var/run/qemu/${vm}.pid -daemonize -nographic


qemu_vm1="-enable-kvm -m 512 -hda /dev/mapper/vg0-vm1 -net nic,macaddr=DE:AD:BE:EF:E0:00 \
 -net tap,ifname=tap0 -serial telnet:localhost:7000,server,nowait,nodelay \
 -monitor telnet:localhost:7100,server,nowait,nodelay -vnc :0"

qemu_vm1_haltcmd="echo 'system_powerdown' | nc.openbsd localhost 7100" # or netcat/ncat

# You can use other ways to shutdown your VM correctly
#qemu_vm1_haltcmd="ssh powermanager@vm1 sudo poweroff"

# By default rc-script will wait 30 seconds before killing VM. Here you can change this timeout.

qemu_vm2="-enable-kvm -m 512 -hda /srv/kvm/vm2.img -net nic,macaddr=DE:AD:BE:EF:E0:01 \
 -net tap,ifname=tap1 -serial telnet:localhost:7001,server,nowait,nodelay \
 -monitor telnet:localhost:7101,server,nowait,nodelay -vnc :1"

qemu_vm2_haltcmd="echo 'system_powerdown' | nc.openbsd localhost 7101"


. /etc/rc.conf
. /etc/rc.d/functions

[ -f /etc/conf.d/qemu.conf ] && source /etc/conf.d/qemu.conf

QEMU_DEFAULT_FLAGS='-name ${vm} -pidfile ${PIDDIR}/${vm}.pid -daemonize -nographic'

case "$1" in
    [ -d "${PIDDIR}" ] || mkdir -p "${PIDDIR}"
    for vm in "${QEMU_MACHINES[@]}"; do
       if [ "${vm}" = "${vm#!}" ]; then
         stat_busy "Starting QEMU VM: ${vm}"
         eval vm_cmdline="\$qemu_${vm}"
         eval vm_type="\$qemu_${vm}_type"

         if [ -n "${vm_type}" ]; then

         eval "qemu_flags=\"${QEMU_DEFAULT_FLAGS}\""

         ${vm_cmd} ${qemu_flags} ${vm_cmdline} >/dev/null
         if [  $? -gt 0 ]; then
    add_daemon qemu

    for vm in "${QEMU_MACHINES[@]}"; do
      if [ "${vm}" = "${vm#!}" ]; then
        # check pidfile presence and permissions
        if [ ! -r "${PIDDIR}/${vm}.pid" ]; then

        stat_busy "Stopping QEMU VM: ${vm}"

        eval vm_haltcmd="\$qemu_${vm}_haltcmd"
        eval vm_haltcmd_wait="\$qemu_${vm}_haltcmd_wait"
        vm_pid=$(cat ${PIDDIR}/${vm}.pid)
        # check process existence
        if ! kill -0 ${vm_pid} 2>/dev/null; then
          rm -f "${PIDDIR}/${vm}.pid"

        # Try to shutdown VM safely
        if [ -n "${vm_haltcmd}" ]; then
          eval ${vm_haltcmd} >/dev/null

          while [ "${_w}" -lt "${vm_haltcmd_wait}" ]; do
            sleep 1
            if ! kill -0 ${vm_pid} 2>/dev/null; then
              # no such process
            _w=$((_w + 1))

          # No haltcmd - kill VM unsafely

        if [ -n "${_vm_running}" ]; then
            # kill VM unsafely
            kill ${vm_pid} 2>/dev/null
            sleep 1

        # report status
        if kill -0 ${vm_pid} 2>/dev/null; then
          # VM is still alive
          #kill -9 ${vm_pid}

        # remove pidfile
        rm -f "${PIDDIR}/${vm}.pid"
    rm_daemon qemu

    $0 stop
    sleep 1
    $0 start

    echo "usage: $0 {start|stop|restart}"


See also