34 Running virtual machines with qemu-system-ARCH #

You can specify the type of the emulated machine. Run qemu-system-ARCH -M help to view a list of supported machine types.

Note: ISA-PC

The machine type isapc: ISA-only-PC is unsupported.

To specify the type of the processor (CPU) model, run qemu-system-ARCH -cpu MODEL. Use qemu-system-ARCH -cpu help to view a list of supported CPU models.

The following is a list of most commonly used options while launching qemu from command line. To see all options available refer to qemu-doc man page.

The following is an example of a working qemu-system-ARCH command line:

> sudo qemu-system-x86_64 \
 -name "SLES 15.4" \
 -M pc-i440fx-2.7 -m 512 \
 -machine accel=kvm -cpu kvm64 -smp 2 \
 -drive format=raw,file=/images/sles.raw

Figure 34.1: QEMU window with SLES as VM Guest #

  

A Sparse image file is a type of disk image file that grows in size as the user adds data to it, taking up only as much disk space as is stored in it. For example, if you copy 1 GB of data inside the sparse disk image, its size grows by 1 GB. If you then delete for example 500 MB of the data, the image size does not by default decrease as expected.

That is why the discard=on option is introduced on the KVM command line. It tells the hypervisor to automatically free the “holes” after deleting data from the sparse guest image. Note that this option is valid only for the if=scsi drive interface:

> sudo qemu-system-x86_64 [...] -drive format=img_format,file=/path/to/file.img,if=scsi,discard=on

Important: Support status

if=scsi is not supported. This interface does not map to virtio-scsi, but rather to the lsi SCSI adapter.

IOThreads are dedicated event loop threads for virtio devices to perform I/O requests in order to improve scalability, especially on an SMP VM Host Server with SMP VM Guests using many disk devices. Instead of using QEMU's main event loop for I/O processing, IOThreads allow spreading I/O work across multiple CPUs and can improve latency when properly configured.

IOThreads are enabled by defining IOThread objects. virtio devices can then use the objects for their I/O event loops. Many virtio devices can use a single IOThread object, or virtio devices and IOThread objects can be configured in a 1:1 mapping. The following example creates a single IOThread with ID iothread0 which is then used as the event loop for two virtio-blk devices.

> sudo qemu-system-x86_64 [...] -object iothread,id=iothread0\
-drive if=none,id=drive0,cache=none,aio=native,\
format=raw,file=filename -device virtio-blk-pci,drive=drive0,scsi=off,\
iothread=iothread0 -drive if=none,id=drive1,cache=none,aio=native,\
format=raw,file=filename -device virtio-blk-pci,drive=drive1,scsi=off,\
iothread=iothread0 [...]

The following qemu command line example illustrates a 1:1 virtio device to IOThread mapping:

> sudo qemu-system-x86_64 [...] -object iothread,id=iothread0\
-object iothread,id=iothread1 -drive if=none,id=drive0,cache=none,aio=native,\
format=raw,file=filename -device virtio-blk-pci,drive=drive0,scsi=off,\
iothread=iothread0 -drive if=none,id=drive1,cache=none,aio=native,\
format=raw,file=filename -device virtio-blk-pci,drive=drive1,scsi=off,\
    iothread=iothread1 [...]

For better performance of I/O-intensive applications, a new I/O path was introduced for the virtio-blk interface in kernel version 3.7. This bio-based block device driver skips the I/O scheduler, and thus shortens the I/O path in guest and has lower latency. It is especially useful for high-speed storage devices, such as SSD disks.

The driver is disabled by default. To use it, do the following:

Tip: Bio-based driver on slow devices

The bio-based virtio-blk driver does not help on slow devices such as spin hard disks. The reason is that the benefit of scheduling is larger than what the shortened bio path offers. Do not use the bio-based driver on slow devices.

QEMU now integrates with libiscsi. This allows QEMU to access iSCSI resources directly and use them as virtual machine block devices. This feature does not require any host iSCSI initiator configuration, as is needed for a libvirt iSCSI target based storage pool setup. Instead it directly connects guest storage interfaces to an iSCSI target LUN by means of the user space library libiscsi. iSCSI-based disk devices can also be specified in the libvirt XML configuration.

Note: RAW image format

This feature is only available using the RAW image format, as the iSCSI protocol has some technical limitations.

The following is the QEMU command line interface for iSCSI connectivity.

Note: virt-manager limitation

The use of libiscsi based storage provisioning is not yet exposed by the virt-manager interface, but instead it would be configured by directly editing the guest xml. This new way of accessing iSCSI based storage is to be done at the command line.

> sudo qemu-system-x86_64 -machine accel=kvm \
  -drive file=iscsi://192.168.100.1:3260/iqn.2016-08.com.example:314605ab-a88e-49af-b4eb-664808a3443b/0,\
  format=raw,if=none,id=mydrive,cache=none \
  -device ide-hd,bus=ide.0,unit=0,drive=mydrive ...

Here is an example snippet of guest domain xml which uses the protocol based iSCSI:

<devices>
...
  <disk type='network' device='disk'>
    <driver name='qemu' type='raw'/>
    <source protocol='iscsi' name='iqn.2013-07.com.example:iscsi-nopool/2'>
      <host name='example.com' port='3260'/>
    </source>
    <auth username='myuser'>
      <secret type='iscsi' usage='libvirtiscsi'/>
    </auth>
    <target dev='vda' bus='virtio'/>
  </disk>
</devices>

Contrast that with an example which uses the host based iSCSI initiator which virt-manager sets up:

<devices>
...
  <disk type='block' device='disk'>
    <driver name='qemu' type='raw' cache='none' io='native'/>
    <source dev='/dev/disk/by-path/scsi-0:0:0:0'/>
    <target dev='hda' bus='ide'/>
    <address type='drive' controller='0' bus='0' target='0' unit='0'/>
  </disk>
  <controller type='ide' index='0'>
    <address type='pci' domain='0x0000' bus='0x00' slot='0x01'
             function='0x1'/>
  </controller>
</devices>

RADOS Block Devices (RBD) store data in a Ceph cluster. They allow snapshotting, replication, and data consistency. You can use an RBD from your KVM-managed VM Guests similarly to how you use other block devices.

QEMU uses -vga to define a video card used to display VM Guest graphical output. The -vga option understands the following values:

The following options affect the way VM Guest graphical output is displayed.

SUSE currently supports the following types of USB devices: disk, host, serial, braille, net, mouse, and tablet.

This section shows several examples on how to set up user-mode networking with QEMU.

> sudo qemu-system-x86_64 [...] \
-netdev user1,id=hostnet0 \
-device virtio-net-pci,netdev=hostnet0,vlan=12,name=user_net13,restrict=yes4

> sudo qemu-system-x86_64 [...] \
-netdev user,id=hostnet0 \
-device virtio-net-pci,netdev=hostnet0,net=10.2.0.0/81,host=10.2.0.62,\
dhcpstart=10.2.0.203,hostname=tux_kvm_guest4

> sudo qemu-system-x86_64 [...] \
-netdev user,id=hostnet0 \
-device virtio-net-pci,netdev=hostnet0,tftp=/images/tftp_dir1,\
bootfile=/images/boot/pxelinux.02

> sudo qemu-system-x86_64 [...] \
-netdev user,id=hostnet0 \
-device virtio-net-pci,netdev=hostnet0,hostfwd=tcp::2222-:22

> ssh qemu_host -p 2222

Use the following example script to connect VM Guest to the newly created bridge interface br0. Several commands in the script are run via the sudo mechanism because they require root privileges.

Tip: Required software

To manage a network bridge, you need to have the tunctl package installed.

#!/bin/bash
bridge=br01
tap=$(sudo tunctl -u $(whoami) -b)2
sudo ip link set $tap up3
sleep 1s4
sudo ip link add name $bridge type bridge
sudo ip link set $bridge up
sudo ip link set $tap master $bridge5
qemu-system-x86_64 -machine accel=kvm -m 512 -hda /images/sles_base.raw \
 -netdev tap,id=hostnet0 \
 -device virtio-net-pci,netdev=hostnet0,vlan=0,macaddr=00:16:35:AF:94:4B,\
 ifname=$tap6,script=no7,downscript=no
sudo ip link set $tap nomaster8
sudo ip link set $tap down9
sudo tunctl -d $tap10

Another way to connect VM Guest to a network through a network bridge is by means of the qemu-bridge-helper helper program. It configures the TAP interface for you, and attaches it to the specified bridge. The default helper executable is /usr/lib/qemu-bridge-helper. The helper executable is setuid root, which is only executable by the members of the virtualization group (kvm). Therefore the qemu-system-ARCH command itself does not need to be run under root privileges.

The helper is automatically called when you specify a network bridge:

qemu-system-x86_64 [...] \
 -netdev bridge,id=hostnet0,vlan=0,br=br0 \
 -device virtio-net-pci,netdev=hostnet0

You can specify your own custom helper script that will take care of the TAP device (de)configuration, with the helper=/path/to/your/helper option:

qemu-system-x86_64 [...] \
 -netdev bridge,id=hostnet0,vlan=0,br=br0,helper=/path/to/bridge-helper \
 -device virtio-net-pci,netdev=hostnet0

Tip

To define access privileges to qemu-bridge-helper, inspect the /etc/qemu/bridge.conf file. For example the following directive

allow br0

allows the qemu-system-ARCH command to connect its VM Guest to the network bridge br0.