Once you have a virtual disk image ready (for more information on disk images, see Section 12.2, “Managing Disk Images with qemu-img”), it is time to start the related virtual machine. Section 12.1, “Basic Installation with qemu-kvm” introduced simple commands to install and run a VM Guest. This chapter focuses on a more detailed explanation of qemu-kvm usage, and shows solutions of more specific tasks. For a complete list of qemu-kvm's options, see its manual page (man 1 qemu-kvm).
The qemu-kvm command uses the following syntax:
qemu-kvm understands a large number of options. Most of them define parameters of the emulated hardware, while others affect more general emulator behavior. If you do not supply any options, default values are used, and you need to supply the path to a disk image to be run.
Path to the disk image holding the guest system you want to virtualize.
qemu-kvm supports a large number of image formats.
Use qemu-img --help to list them. If you do not
supply the path to a disk image as a separate argument, you have to use
This section introduces general qemu-kvm options and options related to the basic emulated hardware, such as virtual machine's processor, memory, model type, or time processing methods.
Specifies the name of the running guest system. The name is displayed in the window caption and also used for the VNC server.
Specifies the order in which the defined drives will be booted. Drives are represented by letters, where 'a' and 'b' stands for the floppy drives 1 and 2, 'c' stands for the first hard disk, 'd' stands for the first CD-ROM drive, and 'n' to 'p' stand for Ether-boot network adapters.
qemu-kvm [...] -boot order=ndc first
tries to boot from network, then from the first CD-ROM drive, and
finally from the first hard disk.
Stores the QEMU's process identification number (PID) in a file. This is useful if you run QEMU from a script.
By default QEMU creates basic virtual devices even if you do not specify them on the command line. This option turns this feature off, and you must specify every single device manually, including graphical and network cards, parallel or serial ports, or virtual consoles. Even QEMU monitor is not attached by default.
'Daemonizes' the QEMU process after it is started. QEMU will detach from the standard input and standard output after it is ready to receive connections on any of its devices.
Specifies the type of the emulated machine. Run qemu-kvm -M ? to view a list of supported machine types.
tux@venus:~> qemu-kvm -M ? Supported machines are: pc Standard PC (alias of pc-0.12) pc-0.12 Standard PC (default) pc-0.11 Standard PC, qemu 0.11 pc-0.10 Standard PC, qemu 0.10 isapc ISA-only PC
Currently, SUSE supports only
Specifies how many megabytes are used for the virtual RAM size. Default is 512 MB.
Specifies a paravirtualized device to dynamically change the amount
of virtual RAM memory assigned to VM Guest. The top limit is the
amount of memory specified with
Specifies the type of the processor (CPU) model. Run qemu-kvm -cpu ? to view a list of supported CPU models.
tux@venus:~> qemu-kvm -cpu ? x86 qemu64 x86 phenom x86 core2duo x86 kvm64 x86 qemu32 x86 coreduo x86 486 x86 pentium x86 pentium2 x86 pentium3 x86 athlon x86 n270
Specifies how many CPUs will be emulated. QEMU supports up to 255 CPUs on the PC platform (up to 64 with KVM acceleration used). This option also takes other CPU-related parameters, such as number of sockets, number of cores per socket, or number of threads per core.
Following is an example of a working qemu-kvm command line:
qemu-kvm -name "SLES 11 SP1" -M pc-0.12 -m 512 -cpu kvm64 \ -smp 2 /images/sles11sp1.raw
Disables ACPI support. Try to use it if VM Guest reports problems with ACPI interface.
QEMU starts with CPU stopped. To start CPU, enter
c in QEMU monitor. For more information, see
Chapter 14, Administrating Virtual Machines with QEMU Monitor.
Instead of entering the devices configuration options on the command
line each time you want to run VM Guest, qemu-kvm
can read it from a file which was either previously saved with
-writeconfig or edited manually.
Dumps the current virtual machine devices configuration to a text
file. It can be consequently re-used with the
tux@venus:~> qemu-kvm -name "SLES 11 SP1" -M pc-0.12 -m 512 -cpu kvm64 \ -smp 2 /images/sles11sp1.raw -writeconfig /images/sles11sp1.cfg (exited) tux@venus:~> more /images/sles11sp1.cfg # qemu config file [drive] index = "0" media = "disk" file = "/images/sles11sp1_base.raw"
This way you can effectively manage the configuration of your virtual machines' devices in a well-arranged way.
Specifies the way the RTC is handled inside a VM Guest. By default, the clock of the guest is derived from that of the host system. Therefore, it is recommended that the host system clock is synchronized with an accurate external clock (for example, via NTP service).
If you need to isolate the VM Guest clock from the host one, specify
clock=vm instead of the default
You can also specify a 'starting point' for VM Guest clock with the
qemu-kvm [...] -rtc clock=vm,base=2010-12-03T01:02:00
Instead of a timestamp, you can specify
localtime. The former instructs VM Guest to start
at the current UTC value (Coordinated Universal Time, see
http://en.wikipedia.org/wiki/UTC), while the latter
applies the local time setting.
QEMU virtual machines emulate all devices needed to run a VM Guest. QEMU supports, for example, several types of network cards, block devices (hard and removable drives), USB devices, character devices (serial and parallel ports), or multimedia devices (graphic and sound cards). For satisfactory operation and performance of the virtual machine, some or all of these devices must be configured correctly. This section introduces options to configure various types of supported devices.
Block devices are vital for virtual machines. In general, these are fixed or removable storage media usually referred to as 'drives'. One of the connected hard drives typically holds the guest operating system to be virtualized.
Virtual machine drives are defined with
option uses many suboptions, some of which are described in this
section. For their complete list, see the manual page (
Sub-options for the
Specifies the path to the disk image which will be used with this drive. If not specified, an empty (removable) drive is assumed.
Specifies the type of interface to which the drive is connected.
virtio are supported by SUSE.
virtio defines a paravirtualized disk driver.
Specifies the index number of a connector on the disk interface (see
if option) where the drive is connected. If
not specified, the index is automatically incremented.
Specifies the type of the media. Can be
hard disks, or
cdrom for removable CD-ROM drives.
Specifies the format of the connected disk image. If not specified,
the format is autodetected. Currently, SUSE supports
Specifies the caching method for the drive. Possible values are
none. For the
qcow2 image format, choose
writeback if you care about performance.
none disables the host page cache and, therefore,
is the safest option. Default is
To simplify defining of block devices, QEMU understands several
shortcuts which you may find handy when entering the
You can use
qemu-kvm -cdrom /images/cdrom.iso
qemu-kvm -drive file=/images/cdrom.iso,index=2,media=cdrom
qemu-kvm -hda /images/imagei1.raw -hdb /images/image2.raw -hdc \ /images/image3.raw -hdd /images/image4.raw
qemu-kvm -drive file=/images/image1.raw,index=0,media=disk \ -drive file=/images/image2.raw,index=1,media=disk \ -drive file=/images/image3.raw,index=2,media=disk \ -drive file=/images/image4.raw,index=3,media=disk
|Using Host Drives Instead of Images|
Normally you will use disk images (see Section 12.2, “Managing Disk Images with qemu-img”) as disk drives of the virtual machine. However, you can also use existing VM Host Server disks, connect them as drives, and access them from VM Guest. Use the host disk device directly instead of disk image filenames.
To access the host CD-ROM drive, use
qemu-kvm [...] -drive file=/dev/cdrom,media=cdrom
To access the host hard disk, use
qemu-kvm [...] -drive file=/dev/hdb,media=disk
When accessing the host hard drive from VM Guest, always make sure the access is read-only. You can do so by modifying the host device permissions.
This section describes QEMU options affecting the type of the emulated video card and the way VM Guest graphical output is displayed.
-vga to define a video card used to
display VM Guest graphical output. The
understands the following values:
Disables video cards on VM Guest (no video card is emulated). You can still access the running VM Guest via the QEMU monitor and the serial console.
Emulates a standard VESA 2.0 VBE video card. Use it if you intend to use high display resolution on VM Guest.
Emulates Cirrus Logic GD5446 video card. Good choice if you insist on high compatibility of the emulated video hardware. Most operating systems (even Windows 95) recognize this type of card.
For best video performance with the
The following options affect the way VM Guest graphical output is displayed.
Disables QEMU's graphical output. The emulated serial port is redirected to the console.
After starting the virtual machine with
H in the virtual console to view the list of other
useful shortcuts, for example, to toggle between the console and the
tux@venus:~> qemu-kvm -hda /images/sles11sp1_base.raw -nographic C-a h print this help C-a x exit emulator C-a s save disk data back to file (if -snapshot) C-a t toggle console timestamps C-a b send break (magic sysrq) C-a c switch between console and monitor C-a C-a sends C-a (pressed C-a c) QEMU 0.12.5 monitor - type 'help' for more information (qemu)
Disables decorations for the QEMU window. Convenient for dedicated desktop workspace.
Starts QEMU graphical output in full screen mode.
Disables the 'close' button of QEMU window and prevents it from being closed by force.
By default QEMU window releases the 'captured' mouse after
pressed. You can change the key combination to either
-alt-grab), or Right Ctrl
There are basically two ways to create USB devices usable by the
VM Guest in KVM: you can either emulate new USB devices inside a
VM Guest, or assign an existing host USB device to a VM Guest. To use
USB devices in QEMU you first need to enable the generic USB driver
-usb option. Then you can specify individual
devices with the
Although QEMU supports much more types of USB devices, SUSE
currently only supports the types
Types of USB devices for the
Emulates a virtual USB mouse. This option overrides the default PS/2
mouse emulation. The following example shows the hardware status of
a mouse on VM Guest started with
qemu-kvm [...] -usbdevice
tux@venus:~> hwinfo --mouse 20: USB 00.0: 10503 USB Mouse [Created at usb.122] UDI: /org/freedesktop/Hal/devices/usb_device_627_1_1_if0 [...] Hardware Class: mouse Model: "Adomax QEMU USB Mouse" Hotplug: USB Vendor: usb 0x0627 "Adomax Technology Co., Ltd" Device: usb 0x0001 "QEMU USB Mouse" [...]
Emulates a pointer device that uses absolute coordinates (such as touchscreen). This option overrides the default PS/2 mouse emulation. The tablet device is useful if you are viewing VM Guest via the VNC protocol. See Section 13.5, “Viewing a VM Guest with VNC” for more information.
To assign an existing host USB device to a VM Guest, you need to find out its host bus and device ID.
tux@vmhost:~> lsusb [...] Bus 002 Device 005: ID 12d1:1406 Huawei Technologies Co., Ltd. E1750 [...]
In the above example, we want to assign a USB stick connected to the host's USB bus number 2 with device number 5. Now run the VM Guest with the following additional options:
qemu-kvm [...] -usb -device usb-host,hostbus=2,hostaddr=5
After the guest is booted, check that the assigned USB device is present on it.
tux@vmguest:~> lsusb [...] Bus 001 Device 002: ID 12d1:1406 Huawei Technologies Co., Ltd. E1750 [...]
The guest operating system must take care of mounting the assigned USB device so that it is accessible for the user.
PCI Pass-Through is a technique to give your VM Guest exclusive access to a PCI device.
To make use of PCI Pass-Through, your motherboard chipset, BIOS, and CPU must have support for AMD's IOMMU (or VT-d in Intel speak) virtualization technology. To make sure that your computer supports this feature, ask your supplier specifically to deliver a system that supports PCI Pass-Through.
Assignment of graphics cards is not supported by SUSE.
Procedure 13.1. Configuring PCI Pass-Through
Make sure that
CONFIG_DMAR_DEFAULT_ON is set in the
host's running Kernel:
grep CONFIG_DMAR_DEFAULT_ON /boot/config-`uname -r`
If this option is not set, edit your boot loader configuration and add
intel_iommu=on (Intel machines) or
iommu=pt iommu=1 (AMD machines). Then reboot the
Check that IOMMU is actively enabled and recognized on the host. Run dmesg | grep -e DMAR -e IOMMU on Intel machines, or dmesg | grep AMD-Vi on AMD machines. If you get no output, check carefully if your hardware supports IOMMU (VT-d) and check that it has been enabled in BIOS.
Identify the host PCI device to assign to the guest.
tux@vmhost:~> lspci -nn [...] 00:1b.0 Audio device : Intel Corporation 82801H (ICH8 Family) \ HD Audio Controller [8086:284b] (rev 02) [...]
Note down the device (
00:1b.0) and vendor
Unbind the device from host Kernel driver and bind it to the PCI stub driver.
tux@vmhost:~> modprobe pci_stub tux@vmhost:~> echo "8086 284b" > /sys/bus/pci/drivers/pci-stub/new_id tux@vmhost:~> echo "0000:00:1b.0" > /sys/bus/pci/devices/0000:00:1b.0/driver/unbind tux@vmhost:~> echo "0000:00:1b.0" > /sys/bus/pci/drivers/pci-stub/bind
Now run the VM Guest with the PCI device assigned.
qemu-kvm [...] -device pci-assign,host=00:1b.0
If the PCI device shares IRQ with other devices, it cannot be assigned to a VM Guest.
KVM also supports PCI device hotplugging to a VM Guest. To achieve this, you need to switch to a QEMU monitor (see Chapter 14, Administrating Virtual Machines with QEMU Monitor for more information) and issue the following commands:
-chardev to create a new character device. The
option uses the following general syntax:
qemu-kvm [...] -chardev
backend_type can be one of
parport. All character devices must have a unique
identification string up to 127 characters long. It is used to identify
the device in other related directives. For the complete description of
all backend's suboptions, see the manual page (man 1
qemu-kvm). A brief description of the available
Creates an empty device which outputs no data and drops any data it receives.
Connects to QEMU's process standard input and standard output.
Creates a two-way stream socket. If
is specified, a Unix socket is created:
qemu-kvm [...] -chardev \ socket,id=unix_socket1,path=/tmp/unix_socket1,server
server suboption specifies that the
socket is a listening socket.
port is specified, a TCP socket is
qemu-kvm [...] -chardev \ socket,id=tcp_socket1,host=localhost,port=7777,server,nowait
The command creates a local listening (
socket on port 7777. QEMU will not block waiting for a client to
connect to the listening port (
Sends all network traffic from VM Guest to a remote host over the UDP protocol.
qemu-kvm [...] -chardev udp,id=udp_fwd,host=mercury.example.com,port=7777
The command binds port 7777 on the remote host mercury.example.com and sends VM Guest network traffic there.
Creates a new QEMU text console. You can optionally specify the dimensions of the virtual console:
qemu-kvm [...] -chardev vc,id=vc1,width=640,height=480 -mon chardev=vc1
The command creates a new virtual console called
vc1 of the specified size, and connects the QEMU
monitor to it.
Logs all traffic from VM Guest to a file on VM Host Server. The
path is required and will be created if it does
qemu-kvm [...] -chardev file,id=qemu_log1,path=/var/log/qemu/guest1.log
By default QEMU creates a set of character devices for serial and parallel ports, and a special console for QEMU monitor. You can, however, create your own character devices and use them for just mentioned purposes. The following options will help you:
Redirects the VM Guest's virtual serial port to a character device
char_dev on VM Host Server. By default, it is a
virtual console (
vc) in graphical mode, and
stdio in non-graphical mode. The
-serial understands many suboptions. See the
manual page man 1 qemu-kvm for their complete
You can emulate up to 4 serial ports. Use
none to disable all serial ports.
Redirects the VM Guest's parallel port to a
device. This option supports the same
With SUSE Linux Enterprise Server as a VM Host Server,
you can directly use the hardware parallel port devices
You can emulate up to 3 parallel ports. Use
none to disable all parallel ports.
Redirects the QEMU monitor to a character device
char_dev on VM Host Server. This option supports
the same devices as
-serial. By default, it is a
virtual console (
vc) in a graphical mode, and
stdio in non-graphical mode.
-net option to define a network interface and
a specific type of networking for your VM Guest. Currently, SUSE
supports the following options:
tap. For a complete list of
suboptions, see the manual page (man 1 qemu-kvm).
Disables a network card emulation on VM Guest. Only the loopback
lo network interface is available.
Creates a new Network Interface Card (NIC) and connects it to a specified Virtual Local Area Network (VLAN). For more information, see Section 13.4.1, “Defining a Network Interface Card”
Specifies a user-mode networking. For more information , see Section 13.4.2, “User-mode Networking”.
Specifies a bridged or routed networking. For more information, see Section 13.4.3, “Bridged Networking”.
-net nic to add a new emulated network card:
Connects the network interface to VLAN number 1. You can specify your own number, it is mainly useful for identification purpose. If you omit this suboption, QEMU uses the default 0.
Specifies the Media Access Control (MAC) address for the network card. It is a unique identifier and you are advised to always specify it. If not, QEMU supplies its own default MAC address and creates a possible MAC address conflict within the related VLAN.
Specifies the model of the network card. Use
Currently, SUSE supports the models
-net user option instructs QEMU to use a
user-mode networking. This is the default if no networking mode is
selected. Therefore, these command lines are equivalent:
qemu-kvm -hda /images/sles11sp1_base.raw
qemu-kvm -hda /images/sles11sp1_base.raw -net nic -net user
This mode is useful if you want to allow the VM Guest to access the external network resources, such as Internet. By default, no incoming traffic is permitted and therefore, the VM Guest is not visible to other machines on the network. No administrator privileges are required in this networking mode. The user-mode is also useful to do a 'network-booting' on your VM Guest from a local directory on VM Host Server.
The VM Guest allocates an IP address from a virtual DHCP server. VM Host Server (the DHCP server) is reachable at 10.0.2.2, while the IP address range for allocation starts from 10.0.2.15. You can use ssh to connect to VM Host Server at 10.0.2.2, and scp to copy files back and forth.
This section shows several examples on how to set up user-mode networking with QEMU.
Example 13.1. Restricted User-mode Networking
Specifies user-mode networking.
Connect to VLAN number 1. If omitted, defaults to 0.
Specifies a human readable name of the network stack. Useful when identifying it in the QEMU monitor.
Isolates VM Guest. It will not be able to communicate with VM Host Server and no network packets will be routed to the external network.
Example 13.2. User-mode Networking with Custom IP Range
Specifies the IP address of the network that VM Guest sees and optionally the netmask. Default is 10.0.2.0/8.
Specifies the VM Host Server IP address that VM Guest sees. Default is 10.0.2.2.
Specifies the first of the 16 IP addresses that the built-in DHCP server can assign to VM Guest. Default is 10.0.2.15.
Specifies the hostname that the built-in DHCP server will assign to VM Guest.
Example 13.3. User-mode Networking with Network-boot and TFTP
Activates a built-in TFTP (a file transfer protocol with the functionality of a very basic FTP) server. The files in the specified directory will be visible to a VM Guest as the root of a TFTP server.
Broadcasts the specified file as a BOOTP (a network protocol which
offers an IP address and a network location of a boot image, often
used in diskless workstations) file. When used together with
Example 13.4. User-mode Networking with Host Port Forwarding
qemu-kvm [...] -net user,hostfwd=tcp::2222-:22
Forwards incoming TCP connections to the port 2222 on the host to the
port 22 (
SSH) on VM Guest.
sshd is running on
VM Guest, enter
ssh qemu_host -p 2222
qemu_host is the hostname or IP address of
the host system, to get a
prompt from VM Guest.
-net tap option, QEMU creates a network
bridge by connecting the host TAP network device to a specified VLAN of
VM Guest. Its network interface is then visible to the rest of the
network. This method does not work by default and has to be explicitly
First, create a network bridge and add a VM Host Server physical network
eth0) to it:
Start YaST Control Center and select+ .
Clickand select from the drop-down list in the window. Click .
Choose whether you need a dynamically or statically assigned IP address, and fill the related network settings if applicable.
In thepane, select the Ethernet device to add to the bridge.
Click. When asked about adapting an already configured device, click .
Clickto apply the changes. Check if the bridge is created:
tux@venus:~> brctl show bridge name bridge id STP enabled interfaces br0 8000.001676d670e4 no eth0
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
Make sure the
#!/bin/bash bridge=br0 tap=$(sudo tunctl -u $(whoami) -b) sudo ip link set $tap up sleep 1s sudo brctl addif $bridge $tap qemu-kvm -m 512 -hda /images/sles11sp1_base.raw \ -net nic,vlan=0,model=virtio,macaddr=00:16:35:AF:94:4B \ -net tap,vlan=0,ifname=$tap,script=no,downscript=no sudo brctl delif $bridge $tap sudo ip link set $tap down sudo tunctl -d $tap
Name of the bridge device.
Prepare a new TAP device and assign it to the user who runs the script. TAP devices are virtual network devices often used for virtualization and emulation setups.
Bring up the newly created TAP network interface.
Make a 1 second pause to make sure the new TAP network interface is really up.
Add the new
Before qemu-kvm connects to a network bridge, it
Deletes the TAP interface from a network bridge
Sets the state of the TAP device to 'down'.
Deconfigures the TAP device.
vhost-net module is used to accelerate
KVM's paravirtualized network drivers. It provides better latency and
greater throughput for network.
To make use of the module, verify that the host's running Kernel has
CONFIG_VHOST_NET turned on or enabled as a module:
grep CONFIG_VHOST_NET /boot/config-`uname -r`
Also verify that the guest's running Kernel has
grep CONFIG_PCI_MSI /boot/config-`uname -r`
If both conditions are met, use the
by starting the guest with the following example command line:
qemu-kvm [...] -netdev tap,id=guest0,vhost=on,script=no -net nic,model=virtio,netdev=guest0,macaddr=00:16:35:AF:94:4B
guest0 is an identification string of the
QEMU normally uses an SDL (a cross-platform multimedia library) window
to display the graphical output of a VM Guest. With the
-vnc option specified, you can make QEMU listen on a
specified VNC display and redirect its graphical output to the VNC
When working with QEMU's virtual machine via VNC session, it is useful
to work with the
Moreover, if you need to use another keyboard layout than the default
The first suboption of
-vnc must be a
display value. The
understands the following display specifications:
Only connections from
host on the display number
display will be accepted. The TCP port on which the
VNC session is then running is normally a 5900 +
display number. If you do not specify
host, connections will be accepted from any host.
The VNC server listens for connections on Unix domain sockets. The
path option specifies the location of the related
The VNC server functionality is initialized, but the server itself is not started. You can start the VNC server later with the QEMU monitor. For more information, see Chapter 14, Administrating Virtual Machines with QEMU Monitor.
tux@venus:~> qemu-kvm [...] -vnc :5 (on the client:) wilber@jupiter:~> vinagre venus:5905 &
The default VNC server setup does not use any form of authentication. In the previous example, any user can connect and view the QEMU VNC Session from any host on the network.
There are several levels of security which you can apply to your VNC client/server connection. You can either protect your connection with a password, use x509 certificates, use SASL authentication, or even combine some of these authentication methods in one QEMU command.
See Section A.2, “Generating x509 Client/Server Certificates” for more information about
the x509 certificates generation. For more information about configuring
x509 certificates on a VM Host Server and the client, see
Section 7.2.2, “Remote TLS/SSL Connection with x509 Certificate (
Section 18.104.22.168, “Configuring the Client and Testing the Setup”.
The Vinagre VNC viewer supports advanced authentication mechanisms.
Therefore, it will be used to view the graphical output of VM Guest in
the following examples. For this example, let us assume that the server
server-key.pem are located in the
/etc/pki/qemu directory on the host, while the
client's certificates are distributed in the following locations on the
Example 13.5. Password Authentication
qemu-kvm [...] -vnc :5,password -monitor stdio
Starts the VM Guest graphical output on VNC display number 5 (usually
port 5905). The
password suboption initializes a
simple password-based authentication method. There is no password set
by default and you have to set one with the change vnc
password command in QEMU monitor:
QEMU 0.12.5 monitor - type 'help' for more information (qemu) change vnc password Password: ****
You need the
-monitor stdio option here, because you
would not be able to manage the QEMU monitor without redirecting its
Example 13.6. x509 Certificate Authentication
The QEMU VNC server can use TLS encryption for the session and x509 certificates for authentication. The server asks the client for a certificate and validates it against the CA certificate. Use this authentication type if your company provides an internal certificate authority.
qemu-kvm [...] -vnc :5,tls,x509verify=/etc/pki/qemu
Example 13.7. x509 Certificate and Password Authentication
You can combine the password authentication with TLS encryption and x509 certificate authentication to create a two-layer authentication model for clients. Remember to set the password in the QEMU monitor after you run the following command:
qemu-kvm [...] -vnc :5,password,tls,x509verify=/etc/pki/qemu -monitor stdio
Example 13.8. SASL Authentication
Simple Authentication and Security Layer (SASL) is a framework for authentication and data security in Internet protocols. It integrates several authentication mechanisms, like PAM, Kerberos, LDAP and more. SASL keeps its own user database, so the connecting user accounts do not need to exist on VM Host Server.
For security reasons, you are advised to combine SASL authentication with TLS encryption and x509 certificates:
qemu-kvm [...] -vnc :5,tls,x509,sasl -monitor stdio
VM Guests usually run in a separate computing space — they are provided their own memory range, dedicated CPUs, and filesystem space. Ability to share parts of VM Host Server's filesystem makes the virtualization environment more flexible by simplifying mutual data exchange. Network filesystems, such as CIFS and NFS, have been the traditional way of sharing folders. But as they are not specifically designed for virtualization purposes, they suffer from major performance and feature issues.
KVM introduces a new and more optimized tool called VirtFS (sometimes referred to as a “filesystem pass-through”). VirtFS uses a paravirtual filesystem driver, which avoids converting the guest application filesystem operations into block device operations, and then again into host filesystem operations. VirtFS uses Plan-9 network protocol for communication between the guest and the host.
You can typically use VirtFS to
access a shared folder from several guests, or to provide guest-to-guest filesystem access.
replace the virtual disk as the root filesystem to which the guest's ramdisk connects to during the guest boot process
provide storage services to different customers from a single host filesystem in a cloud environment
In QEMU, the implementation of VirtFS is facilitated by defining two types of devices:
virtio-9p-pci device which transports protocol
messages and data between the host and the guest.
fsdev device which defines the export filesystem
properties, such as filesystem type and security model.
Example 13.9. Exporting Host's Filesystem with VirtFS
qemu-kvm [...] -fsdev local,id=exp1,path=/tmp/,security_model=mapped -device virtio-9p-pci,fsdev=exp1,mount_tag=v_tmp
Identification of the filesystem to be exported.
Filesystem path on the host to be exported.
Security model to be used —
The exported filesystem ID defined before with
Mount tag used later on the guest to mount the exported filesystem.
Such an exported filesystem can be mounted on the guest like this
mount -t 9p -o trans=virtio v_tmp /mnt
v_tmp is the mount tag defined earlier with
-device mount_tag= and
the mount point where you want to mount the exported filesystem.
Kernel SamePage Merging (KSM) is a Linux Kernel feature which merges identical memory pages from multiple running processes into one memory region. Because KVM guests run as processes under Linux, KSM provides the memory overcommit feature to hypervisors for more efficient use of memory. Therefore, if you need to run multiple virtual machines on a host with limited memory, KSM is the best solution for you.
To make use of KSM, do the following.
Verify that KSM is enabled in your running Kernel:
grep KSM /boot/config-`uname -r` CONFIG_KSM=y
If KSM is enabled in the running Kernel, you will see the following
files under the
ls -l /sys/kernel/mm/ksm total 0 drwxr-xr-x 2 root root 0 Nov 9 07:10 ./ drwxr-xr-x 6 root root 0 Nov 9 07:10 ../ -r--r--r-- 1 root root 4096 Nov 9 07:10 full_scans -r--r--r-- 1 root root 4096 Nov 9 07:10 pages_shared -r--r--r-- 1 root root 4096 Nov 9 07:10 pages_sharing -rw-r--r-- 1 root root 4096 Nov 9 07:10 pages_to_scan -r--r--r-- 1 root root 4096 Nov 9 07:10 pages_unshared -r--r--r-- 1 root root 4096 Nov 9 07:10 pages_volatile -rw-r--r-- 1 root root 4096 Nov 9 07:10 run -rw-r--r-- 1 root root 4096 Nov 9 07:10 sleep_millisecs
Check if KSM feature is turned on.
If the command returns
0, turn KSM on with
echo 1 > /sys/kernel/mm/ksm/run
Now run several VM Guests under KVM and inspect the content of files
pages_shared, for example:
while [ 1 ]; do cat /sys/kernel/mm/ksm/pages_shared; sleep 1; done 13522 13523 13519 13518 13520 13520 13528
For more information on the meaning of the
/sys/kernel/mm/ksm/* files, see