Chapter 13. Running Virtual Machines with qemu-kvm


13.1. Basic qemu-kvm Invocation
13.2. General qemu-kvm Options
13.3. QEMU Virtual Devices
13.4. Networking with QEMU
13.5. Viewing VM Guest with VNC

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 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).

13.1. Basic qemu-kvm Invocation

The qemu-kvm command uses the following syntax:

qemu-kvm options1 disk_img2


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 the -drive file= option.

13.2. General qemu-kvm Options

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.

-name name_of_guest

Specifies the name of the running guest system. The name is displayed in the window caption and also used for the VNC server.

-boot options

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.

For example, 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.

-pidfile fname

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.

13.2.1. Basic Virtual Hardware

-M machine_type

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
mac        Intel-Mac

Currently, Novell supports only the default pc-0.12 machine type.

-m megabytes

Specifies how many megabytes are used for the virtual RAM size. Default is 128 MB.

-balloon virtio

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 -m.

-cpu cpu_model

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

Currently, Novell supports only the kvm64 CPU model.

-smp number_of_cpus

Specifies how many CPUs will be emulated. QEMU supports up to 255 CPUs on the PC platform. 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

Figure 13.1. QEMU Window with SLES 11 SP1 as VM Guest

QEMU Window with SLES 11 SP1 as VM Guest


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.

13.2.2. Storing and Reading Configuration of Virtual Devices

-readconfig cfg_file

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.

-writeconfig cfg_file

Dumps the current virtual machine devices configuration to a text file. It can be consequently re-used with the -readconfig option.

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
tux@venus:~> more /images/sles11sp1.cfg
# qemu config file

  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.

13.2.3. Guest Real-time Clock

-rtc options

Specifies the way the RTC is handled inside VM Guest. By default, the clock of VM 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 clock=host.

You can also specify a 'starting point' for VM Guest clock with the base option:

qemu-kvm [...] -rtc clock=vm,base=2010-12-03T01:02:00

Instead of a timestamp, you can specify utc or localtime. The former instructs VM Guest to start at UTC (Coordinated Universal Time, see, while the latter applies the local time setting.

13.3. QEMU Virtual Devices

QEMU virtual machines emulate all devices needed to run 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.

13.3.1. Block 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 -drive. This option uses many suboptions, some of which are described in this section. For their complete list, see the manual page (man 1 qemu-kvm).

Suboptions for the -drive Option


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. Currently only floppy, ide, or virtio are supported by Novell. virtio defines a paravirtualized disk driver. Default is ide.


Specifies the index number of a connector on the disk interface (see the if option) where the drive is connected. If not specified, the index is automatically incremented.


Specifies the type of the media. Can be disk for 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, Novell supports only the raw format.

boot='on' or 'off'

Specifies whether booting from 'uncommon' devices (such as virtio) is allowed. If not specified, disks connected via virtio interface will refuse to boot.


Specifies the caching method for the drive. Possible values are unsafe, writethrough, writeback, or 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 writethrough.


To simplify defining of block devices, QEMU understands several shortcuts which you may find handy when entering the qemu-kvm command line.

You can use

qemu-kvm -cdrom /images/cdrom.iso

instead of

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

instead of

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
[Tip]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.

13.3.2. Graphic Devices and Display Options

This section describes QEMU options affecting the type of the emulated video card and the way VM Guest graphical output is displayed. Defining Video Cards

QEMU uses -vga to define a video card used to display VM Guest graphical output. The -vga option 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 cirrus type, use 16-bit color depth both on VM Guest and VM Host Server. Display Options

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 -nographic, press +A H in the virtual console to view the list of other useful shortcuts, for example, to toggle between the console and the QEMU monitor.

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

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.

-alt-grab, -ctrl-grab

By default QEMU window releases the 'captured' mouse after + is pressed. You can change the key combination to either ++ (-alt-grab), or Right Ctrl (-ctrl-grab).

13.3.3. USB Devices

To emulate USB devices in QEMU you first need to enable the generic USB driver with the -usb option. Then you can specify individual devices with the -usbdevice option. Although QEMU supports much more types of USB devices, Novell currently only supports the types mouse and tablet.

Types of USB devices for the -usbdevice Option


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 mouse:

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 VM Guest with VNC” for more information.

13.3.4. Character Devices

Use -chardev to create a new character device. The option uses the following general syntax:

qemu-kvm [...] -chardev backend_type,id=id_string

where backend_type can be one of null, socket, udp, msmouse, vc, file, pipe, console, serial, pty, stdio, braille, tty, or 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 man page (man 1 qemu-kvm). A brief description of the available backends follows:


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 path is specified, a Unix socket is created:

qemu-kvm [...] -chardev \

The server suboption specifies that the socket is a listening socket.

If port is specified, a TCP socket is created:

qemu-kvm [...] -chardev \

The command creates a local listening (server) TCP socket on port 7777. QEMU will not block waiting for a client to connect to the listening port (nowait).


Sends all network traffic from VM Guest to a remote host over the UDP protocol.

qemu-kvm [...] -chardev udp,id=udp_fwd,,port=7777

The command binds port 7777 on the remote host 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 not exist.

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:

-serial char_dev

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 list.

You can emulate up to 4 serial ports. Use -serial none to disable all serial ports.

-parallel device

Redirects the VM Guest's parallel port to a device. This option supports the same devices as -serial.


If your VM Host Server is Linux, you can directly use the hardware parallel port devices /dev/parportN where N is the number of the port.

You can emulate up to 3 parallel ports. Use -parallel none to disable all parallel ports.

-monitor char_dev

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.

13.4. Networking with QEMU

Use the -net option to define a network interface and a specific type of networking for your VM Guest. Currently, Novell supports the following options: none, nic, user, and tap. For a complete list of -net suboptions, see the man page (man 1 qemu-kvm).

Supported -net Suboptions


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 networking. For more information, see Section 13.4.3, “Bridged Networking”.

13.4.1. Defining a Network Interface Card

Use -net nic to add a new emulated network card:

qemu-kvm [...] -net nic,vlan=11,macaddr=00:16:35:AF:94:4B2,\


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 -net nic,model=? to get the list of all network card models supported by QEMU on your platform:

Currently, Novell supports the models rtl8139 and virtio.

13.4.2. User-mode Networking

The -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 VM Guest to access the external network resources, such as Internet. By default, no incoming traffic is permitted and therefore, 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, while the IP address range for allocation starts from You can use ssh to connect to VM Host Server at, and scp to copy files back and forth. Command Line Examples

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

Example 13.1. Restricted User-mode Networking

qemu-kvm [...] -net user1,vlan=12,name=user_net13,restrict=yes4


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

qemu-kvm [...] -net user,net=,host=,dhcpstart=,\


Specifies the IP address of the network that VM Guest sees and optionally the netmask. Default is


Specifies the VM Host Server IP address that VM Guest sees. Default is


Specifies the first of the 16 IP addresses that the built-in DHCP server can assign to VM Guest. Default is


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

qemu=kvm [...] -net user,tftp=/images/tftp_dir1,bootfile=/images/boot/pxelinux.02


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 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 tftp, VM Guest can boot from network from the local directory on the host.

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. If sshd is running on VM Guest, enter

ssh qemu_host -p 2222

where qemu_host is the hostname or IP address of the host system, to get a SSH prompt from VM Guest.

13.4.3. Bridged Networking

With the -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 specified.

First, create a network bridge and add a VM Host Server physical network interface (usually eth0) to it:

  1. Start YaST Control Center and select Network Devices+Network Settings.

  2. Click Add and select Bridge from the Device Type drop-down list in the Hardware Dialog window. Click Next.

  3. Choose whether you need a dynamically or statically assigned IP address, and fill the related network settings if applicable.

  4. In the Bridged Devices pane, select the Ethernet device to add to the bridge.

    Figure 13.2. Configuring Network Bridge with YaST

    Configuring Network Bridge with YaST

    Click Next. If asked about adapting already configured device, click Continue.

  5. Click OK to 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 require root privileges.


Make sure the tunctl and bridge-utils packages are installed on VM Host Server. If not, install them with zypper in tunctl bridge-utils.

tap=$(/usr/bin/sudo /bin/tunctl -u $(/usr/bin/whoami) -b)2
/usr/bin/sudo /sbin/ip link set $tap up3
sleep 1s4
/usr/bin/sudo /sbin/brctl addif $bridge $tap5
qemu-kvm -m 512 -hda /images/sles11sp1_base.raw \
-net nic,vlan=0,model=virtio,macaddr=00:16:35:AF:94:4B \
-net tap,vlan=0,ifname=$tap6,script=no7,downscript=no
/usr/bin/sudo /sbin/brctl delif $bridge $tap8
/usr/bin/sudo /sbin/ip link set $tap down9
/usr/bin/sudo /bin/tunctl -d $tap10


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 TAP device to the network bridge br0.


The ifname= suboption specifies the name of the TAP network interface used for bridging.


Before qemu-kvm connects to a network bridge, it checks the script and downscript values. If it finds the specified scripts on the VM Host Server filesystem, it runs the script before it connects to the network bridge and downscript after it exits the network environment. You can use these scripts to first set up and bring up the bridged network devices, and then to deconfigure them. By default, /etc/qemu-ifup and /etc/qemu-ifdown are examined. If script=no and downscript=no are specified, the script execution is disabled and you have to take care manually.


Deletes the TAP interface from a network bridge br0.


Sets the state of the TAP device to 'down'.


Deconfigures the TAP device.

13.5. Viewing VM Guest with VNC

QEMU normally uses an SDL (a cross-platform multimedia library) window to display the graphical output of 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 session.


When working with QEMU's virtual machine via VNC session, it is useful to work with the -usbdevice tablet option.

Moreover, if you need to use another keyboard layout than the default en-us, specify it with the -k option.

The first suboption of -vnc must be a display value. The -vnc option 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 Unix socket.


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 &

Figure 13.3. QEMU VNC Session

QEMU VNC Session

13.5.1. Secure VNC Connections

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 VM Host Server and the client, see Section 7.2.2, “Remote TLS/SSL Connection with x509 Certificate” and Section, “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, lets assume that the server x509 certificates ca-cert.pem, server-cert.pem, and 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 client:


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 input/output.

Figure 13.4. Authentication Dialog in Vinagre

Authentication Dialog in Vinagre

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 TSL 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