QEMU (简体中文)

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翻译状态: 本文是英文页面 QEMU翻译,最后翻译时间:2016-08-25,点击这里可以查看翻译后英文页面的改动。

来自 QEMU 关于页面,

"Qemu是一个广泛使用的开源计算机仿真器和虚拟机"

当作为仿真器时,可以在一种架构(如PC机)下运行另一种架构(如ARM)下的操作系统和程序。通过动态转化,可以获得很高的运行效率。

当 QEME 作为虚拟机时,可以使用 xenkvm 访问 CPU 的扩展功能(HVM),在主机 CPU 上直接执行虚拟客户端的代码,获得接近于真机的性能表现。

Contents

安装

安装 qemu,并根据需要安装下面的可选软件包:

QEMU 图形前端

与其他的虚拟方案如 VirtualBoxVMware不同, QEMU不提供用户图形界面来管理虚拟机 (除了运行虚拟机时出现的窗口), 也没有提供一种持久保存虚拟机设置方式. 必须每一次在命令行上指定所有运行虚拟机的参数,除非你已经创建了一个自定义的脚本来启动你的虚拟机. 有几个程序为 QEMU 提供图形界面:

libvirt 也有支持 QEME 的额外前端界面。

创建新虚拟系统

创建硬盘镜像

提示: QEMU Wikibook 提供了 QEMU 镜像的更多信息。

除非直接从 CD-ROM 或网络引导并且不安装系统到本地,运行 QEMU 时都需要硬盘镜像。硬盘镜像是一个文件,存储虚拟机硬盘上的内容。

一个硬盘镜像可能是 raw镜像, 和客户机器上看到的内容一模一样,主机上占用的空间客户机上的大小一样。这个方式 I/O 效率最高,但是因为客户机器上没使用的空间也被占用,所以有点浪费空间。

另外一种方式是qcow2 格式,仅当客户系统实际写入内容的时候,才会分配镜像空间。对客户机器来说,硬盘大小是完整大小,但是在主机系统上实际仅占用和很小的空间。使用这种方式会影响效率.

QEMU 提供 qemu-img命令创建硬盘镜像.例如创建一个 4 GB raw 格式的镜像:

$ qemu-img create -f raw image_file 4G

您可以用 -f qcow2 改为创建一个 qcow2 磁盘

ddfallocate 也可以创建一个 raw 镜像。

Warning: 如果硬盘镜像存储在 Btrfs 系统上,在创建前请考虑禁用 写时复制

上层存储镜像

可以创建一个基础镜像,and have QEMU keep mutations to this image in an overlay image. This allows you to revert to a previous state of this storage image. You could revert by creating a new overlay image at the time you wish to revert, based on the original backing image.

To create an overlay image, issue a command like:

$ qemu-img create -o backing_file=img1.raw,backing_fmt=raw -f qcow2 img1.cow

After that you can run your QEMU VM as usual (see #Running virtualized system[broken link: invalid section]):

$ qemu-system-i386 img1.cow

The backing image will then be left intact and mutations to this storage will be recorded in the overlay image file.

When the path to the backing image changes, repair is required.

Warning: The backing image's absolute filesystem path is stored in the (binary) overlay image file. Changing the backing image's path requires some effort.

Make sure that the original backing image's path still leads to this image. If necessary, make a symbolic link at the original path to the new path. Then issue a command like:

$ qemu-img rebase -b /new/img1.raw /new/img1.cow

At your discretion, you may alternatively perform an 'unsafe' rebase where the old path to the backing image is not checked:

$ qemu-img rebase -u -b /new/img1.raw /new/img1.cow

调整镜像大小

警告: 调整包含NTFS引导文件系统的镜像将无法启动已安装的操作系统. 完整的解释和解决办法参见 [1].

执行 qemu-imgresize 选项调整硬盘驱动镜像的大小.它适用于 rawqcow2. 例如, 增加镜像 10 GB 大小, 运行:

$ qemu-img resize disk_image +10G

After enlarging the disk image, you must use file system and partitioning tools inside the virtual machine to actually begin using the new space. When shrinking a disk image, you must first reduce the allocated file systems and partition sizes using the file system and partitioning tools inside the virtual machine and then shrink the disk image accordingly, otherwise shrinking the disk image will result in data loss!

准备安装介质

要将操作系统安装到您的磁盘镜像, 你需要操作系统的安装介质 (例如 光盘, USB设备, 或 ISO 镜像). 不要挂载安装介质,因为 QEMU 要直接访问媒体。

Tip: If using an optical disk, 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=cd_image.iso

运行虚拟化的系统

qemu-system-* 程序 (例如 qemu-system-i386qemu-system-x86_64, 取决于客户机架构)用来运行虚拟化的客户机. 用法是:

$ qemu-system-i386 options disk_image

所有 qemu-system-*的选项是相同的,参见 qemu(1) 查看文档和所有选项

默认 QEMU会在窗口中显示虚拟机的视频输出.有一点要记住:当您单击QEMU窗口,鼠标指针被捕获。要放开,按 Ctrl+Alt.

警告: QEMU 不应以 root 身份运行. If you must launch it in a script as root, you should use the -runas option to make QEMU drop root privileges.

启用 KVM

KVM 必须要您处理器和内核支持, 和必要的 kernel modules加载. 更多信息参见 KVM.

要在KVM模式中启动QEMU, 追加 -enable-kvm到启动选项. To check if KVM is enabled for a running VM, enter the QEMU Monitor using Ctrl+Alt+Shift+2, and type info kvm.

Note:
  • If you start your VM with a GUI tool and experience very bad performance, you should check for proper KVM support, as QEMU may be falling back to software emulation.
  • KVM needs to be enabled in order to start Windows 7 and Windows 8 properly without a blue screen.

Enabling IOMMU (Intel VT-d/AMD-Vi) support

Using IOMMU opens to features like PCI passthrough and memory protection from faulty or malicious devices, see wikipedia:Input-output memory management unit#Advantages and Memory Management (computer programming): Could you explain IOMMU in plain English?.

To enable IOMMU:

  1. Ensure that AMD-Vi/Intel VT-d is supported by the CPU and is enabled in the BIOS settings.
  2. Add intel_iommu=on if you have an Intel CPU or amd_iommu=on if you have an AMD CPU, to the kernel parameters.
  3. Reboot and ensure IOMMU is enabled by checking dmesg for DMAR: [0.000000] DMAR: IOMMU enabled
  4. Add iommu=on or q35,iommu=on depending on the -machine, as option.

宿主机和虚拟机数据交互

Network

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 10.0.2.2. 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

QEMU's documentation says it has a "built-in" SMB server, but actually it just starts up Samba with an automatically generated smb.conf file located at /tmp/qemu-smb.pid-0/smb.conf and makes it accessible to the guest at a different IP address (10.0.2.4 by default). This only works for user networking, and this is not 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-system-i386 disk_image -net nic -net user,smb=shared_dir_path

where shared_dir_path 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 10.0.2.4 with the share name "qemu". For example, in Windows Explorer you would go to \\10.0.2.4\qemu.

Note:
  • If you are using sharing options multiple times like -net user,smb=shared_dir_path1 -net user,smb=shared_dir_path2 or -net user,smb=shared_dir_path1,smb=shared_dir_path2 then it will share only the last defined one.
  • If you cannot access the shared folder and the guest system is Windows, check that the NetBIOS protocol is enabled and that a firewall does not block ports used by the NetBIOS protocol.

挂载虚拟硬盘镜像

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 is very likely to occur.

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 disk_image mountpoint

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 disk_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 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 kernel module, and load it with the max_part=15 parameter set. Additionally, the maximum number of loop devices can be controlled with the max_loop parameter.
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 -P disk_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 mountpoint

To mount the disk image with udisksctl, see Udisks#Mount loop devices.

With kpartx

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

# kpartx -a disk_image

This will setup the loopback device and create the necessary partition(s) device(s) in /dev/mapper/.

挂载 qcow2 镜像

你可以使用 qemu-nbd 挂载 qcow2 镜像. 参见 Wikipedia:Qcow#Mounting_qcow2_images.

使用物理分区作为硬盘镜像中的唯一主分区

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 file system 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.
  • You must not mount a file system 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 file system and not as a partitioned device with a MBR. 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 file system as the virtual disk, which will not appear to be partitioned. This is done by issuing a command similar to the following:

Note: In this example, it is the host's images that are being used, not the guest's. If you wish to use the guest's images, either mount /dev/sda3 read-only (to protect the file system from the host) and specify the /full/path/to/images or use some kexec hackery in the guest to reload the guest's kernel (extends boot time).
$ qemu-system-i386 -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 file system 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.

When there are multiple kernel parameters to be passed to the -append option, they need to be quoted using single or double quotes. For example:

... -append 'root=/dev/sda1 console=ttyS0'

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 file system 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 file system 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 us 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 X 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 R to return to the main menu.

Press P 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 now have a partition you can mount directly from your host, as well as part of a QEMU disk image:

$ qemu-system-i386 -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.

网络

The performance of virtual networking should be better with tap devices and bridges than with user-mode networking or vde because 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. See #Installing virtio drivers for more information.

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.

Make sure that each virtual machine has a unique link-level address, but it should always start with 52:54:. Use the following option, replace X with arbitrary hexadecimal digit:

$ qemu-system-i386 -net nic,macaddr=52:54:XX:XX:XX:XX -net vde disk_image

Generating unique link-level addresses can be done in several ways:

  1. Manually specify unique link-level address for each NIC. The benefit is that the DHCP server will assign the same IP address each time the virtual machine is run, but it is unusable for large number of virtual machines.
  2. Generate random link-level address each time the virtual machine is run. Practically zero probability of collisions, but the downside is that the DHCP server will assign a different IP address each time. You can use the following command in a script to generate random link-level address in a macaddr variable:
    printf -v macaddr "52:54:%02x:%02x:%02x:%02x" $(( $RANDOM & 0xff)) $(( $RANDOM & 0xff )) $(( $RANDOM & 0xff)) $(( $RANDOM & 0xff ))
    qemu-system-i386 -net nic,macaddr="$macaddr" -net vde disk_image
  3. Use the following script qemu-mac-hasher.py to generate the link-level address from the virtual machine name using a hashing function. Given that the names of virtual machines are unique, this method combines the benefits of the aforementioned methods: it generates the same link-level address each time the script is run, yet it preserves the practically zero probability of collisions.
    qemu-mac-hasher.py
    #!/usr/bin/env python
    
    import sys
    import zlib
    
    if len(sys.argv) != 2:
        print("usage: %s <VM Name>" % sys.argv[0])
        sys.exit(1)
    
    crc = zlib.crc32(sys.argv[1].encode("utf-8")) & 0xffffffff
    crc = str(hex(crc))[2:]
    print("52:54:%s%s:%s%s:%s%s:%s%s" % tuple(crc))
    

    In a script, you can use for example:

    vm_name="VM Name"
    qemu-system-i386 -name "$vm_name" -net nic,macaddr=$(qemu-mac-hasher.py "$vm_name") -net vde disk_image
    

User-mode networking

By default, without any -netdev 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.

Warning: This only works with the TCP and UDP protocols, so ICMP, including ping, will not work. Do not use ping to test network connectivity.

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, redirecting host ports to the guest (for example to allow SSH connections to the guest) or attaching guests to VLANs 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

Tap devices are a Linux kernel feature that allows you to create virtual 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.

Warning: If you bridge together tap device and some host interface, such as eth0, your virtual machines will appear directly on the external network, which will expose them to possible 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. If the risk is too great, virtual machines have little resources or you set up multiple virtual machines, a better solution might be to use host-only networking and set up NAT. In this case you only need one firewall on the host instead of multiple firewalls for each guest.

As indicated in the user-mode networking section, tap devices offer higher networking performance than user-mode. If the guest OS supports virtio network driver, then the networking performance will be increased considerably as well. Supposing the use of the tap0 device, that the virtio driver is used on the guest, and that no scripts are used to help start/stop networking, next is part of the qemu command one should see:

-net nic,model=virtio -net tap,ifname=tap0,script=no,downscript=no

But if already using a tap device with virtio networking driver, one can even boost the networking performance by enabling vhost, like:

-net nic,model=virtio -net tap,ifname=tap0,script=no,downscript=no,vhost=on

See http://www.linux-kvm.com/content/how-maximize-virtio-net-performance-vhost-net for more information.

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 connected to the bridge, then the virtual machines will be able to talk to each other and the host system. 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.

Tip:
  • If you want to set up IP masquerading, e.g. NAT for virtual machines, see the Internet sharing#Enable NAT page.
  • You may want to have a DHCP server running on the bridge interface to service the virtual network. For example, to use the 172.20.0.1/16 subnet with dnsmasq as the DHCP server:
# ip addr add 172.20.0.1/16 dev br0
# ip link set br0 up
# dnsmasq --interface=br0 --bind-interfaces --dhcp-range=172.20.0.2,172.20.255.254

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.

By default iptables would drop packets in the bridge network. You may need to use such iptables rule to allow packets in a bridged network:

# iptables -I FORWARD -m physdev --physdev-is-bridged -j ACCEPT

Bridged networking using qemu-bridge-helper

Note: This method is available since QEMU 1.1, see http://wiki.qemu.org/Features/HelperNetworking.

This method does not require a start-up script and readily accommodates multiple taps and multiple bridges. It uses /usr/lib/qemu/qemu-bridge-helper binary, which allows creating tap devices on an existing bridge.

Tip: See Network bridge for information on creating bridge.

First, create a configuration file containing the names of all bridges to be used by QEMU:

/etc/qemu/bridge.conf
allow bridge0
allow bridge1
...

Now start the VM. The most basic usage would be:

$ qemu-system-i386 -net nic -net bridge,br=bridge0 [...]

With multiple taps, the most basic usage requires specifying the VLAN for all additional NICs:

$ qemu-system-i386 -net nic -net bridge,br=bridge0 -net nic,vlan=1 -net bridge,vlan=1,br=bridge1 [...]

Creating bridge manually

Tango-edit-clear.pngThis article or section needs language, wiki syntax or style improvements.Tango-edit-clear.png

Reason: This section needs serious cleanup and may contain out-of-date information. (Discuss in Talk:QEMU (简体中文)#)
Tip: Since QEMU 1.1, the network bridge helper can set tun/tap up for you without the need for additional scripting. See #Bridged networking using qemu-bridge-helper.

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.

We will replace the normal Ethernet adapter with a bridge adapter and bind the normal Ethernet adapter to it.

  • Install bridge-utils, which provides brctl to manipulate bridges.
  • Enable IPv4 forwarding:
# sysctl net.ipv4.ip_forward=1

To make the change permanent, change net.ipv4.ip_forward = 0 to net.ipv4.ip_forward = 1 in /etc/sysctl.d/99-sysctl.conf.

  • Load the tun module and configure it to be loaded on boot. See Kernel modules for details.
  • Now create the bridge. See Bridge with netctl for details. Remember to name your bridge as br0, or change the scripts below to your bridge's name.
  • Create the script that QEMU uses to bring up the tap adapter with root:kvm 750 permissions:
/etc/qemu-ifup
#!/bin/sh

echo "Executing /etc/qemu-ifup"
echo "Bringing up $1 for bridged mode..."
sudo /usr/bin/ip link set $1 up promisc on
echo "Adding $1 to br0..."
sudo /usr/bin/brctl addif br0 $1
sleep 2
  • Create the script that QEMU uses to bring down the tap adapter in /etc/qemu-ifdown with root:kvm 750 permissions:
/etc/qemu-ifdown
#!/bin/sh

echo "Executing /etc/qemu-ifdown"
sudo /usr/bin/ip link set $1 down
sudo /usr/bin/brctl delif br0 $1
sudo /usr/bin/ip link delete dev $1
  • Use visudo to add the following to your sudoers file:
Cmnd_Alias      QEMU=/usr/bin/ip,/usr/bin/modprobe,/usr/bin/brctl
%kvm     ALL=NOPASSWD: QEMU
  • You launch QEMU using the following run-qemu script:
run-qemu
#!/bin/bash
USERID=$(whoami)

# Get name of newly created TAP device; see https://bbs.archlinux.org/viewtopic.php?pid=1285079#p1285079
precreationg=$(/usr/bin/ip tuntap list | /usr/bin/cut -d: -f1 | /usr/bin/sort)
sudo /usr/bin/ip tuntap add user $USERID mode tap
postcreation=$(/usr/bin/ip tuntap list | /usr/bin/cut -d: -f1 | /usr/bin/sort)
IFACE=$(comm -13 <(echo "$precreationg") <(echo "$postcreation"))

# This line creates a random MAC address. The downside is the DHCP server will assign a different IP address each time
printf -v macaddr "52:54:%02x:%02x:%02x:%02x" $(( $RANDOM & 0xff)) $(( $RANDOM & 0xff )) $(( $RANDOM & 0xff)) $(( $RANDOM & 0xff ))
# Instead, uncomment and edit this line to set a static MAC address. The benefit is that the DHCP server will assign the same IP address.
# macaddr='52:54:be:36:42:a9'

qemu-system-i386 -net nic,macaddr=$macaddr -net tap,ifname="$IFACE" $*

sudo ip link set dev $IFACE down &> /dev/null
sudo ip tuntap del $IFACE mode tap &> /dev/null

Then to launch a VM, do something like this

$ run-qemu -hda myvm.img -m 512 -vga std
/etc/sysctl.d/10-disable-firewall-on-bridge.conf
net.bridge.bridge-nf-call-ip6tables = 0
net.bridge.bridge-nf-call-iptables = 0
net.bridge.bridge-nf-call-arptables = 0

Run sysctl -p /etc/sysctl.d/10-disable-firewall-on-bridge.conf to apply the changes immediately.

See the libvirt wiki and Fedora bug 512206. If you get errors by sysctl during boot about non-existing files, make the bridge module load at boot. See Kernel modules#Automatic module handling.

Alternatively, you can configure iptables to allow all traffic to be forwarded across the bridge by adding a rule like this:

-I FORWARD -m physdev --physdev-is-bridged -j ACCEPT

Network sharing between physical device and a Tap device through iptables

Merge-arrows-2.pngThis article or section is a candidate for merging with Internet_sharing.Merge-arrows-2.png

Notes: Duplication, not specific to QEMU. (Discuss in Talk:QEMU (简体中文)#)

Bridged networking works fine between a wired interface (Eg. eth0), and it is easy to setup. However if the host gets connected to the network through a wireless device, then bridging is not possible.

See Network bridge#Wireless interface on a bridge as a reference.

One way to overcome that is to setup a tap device with a static IP, making linux automatically handle the routing for it, and then forward traffic between the tap interface and the device connected to the network through iptables rules.

See Internet sharing as a reference.

There you can find what is needed to share the network between devices, included tap and tun ones. The following just hints further on some of the host configurations required. As indicated in the reference above, the client needs to be configured for a static IP, using the IP assigned to the tap interface as the gateway. The caveat is that the DNS servers on the client might need to be manually edited if they change when changing from one host device connected to the network to another.

To allow IP forwarding on every boot, one need to add the following lines to sysctl configuration file inside /etc/sysctl.d:

net.ipv4.ip_forward = 1
net.ipv6.conf.default.forwarding = 1
net.ipv6.conf.all.forwarding = 1

The iptables rules can look like:

# Forwarding from/to outside
iptables -A FORWARD -i ${INT} -o ${EXT_0} -j ACCEPT
iptables -A FORWARD -i ${INT} -o ${EXT_1} -j ACCEPT
iptables -A FORWARD -i ${INT} -o ${EXT_2} -j ACCEPT
iptables -A FORWARD -i ${EXT_0} -o ${INT} -j ACCEPT
iptables -A FORWARD -i ${EXT_1} -o ${INT} -j ACCEPT
iptables -A FORWARD -i ${EXT_2} -o ${INT} -j ACCEPT
# NAT/Masquerade (network address translation)
iptables -t nat -A POSTROUTING -o ${EXT_0} -j MASQUERADE
iptables -t nat -A POSTROUTING -o ${EXT_1} -j MASQUERADE
iptables -t nat -A POSTROUTING -o ${EXT_2} -j MASQUERADE

The prior supposes there are 3 devices connected to the network sharing traffic with one internal device, where for example:

INT=tap0
EXT_0=eth0
EXT_1=wlan0
EXT_2=tun0

The prior shows a forwarding that would allow sharing wired and wireless connections with the tap device.

The forwarding rules shown are stateless, and for pure forwarding. One could think of restricting specific traffic, putting a firewall in place to protect the guest and others. However those would decrease the networking performance, while a simple bridge does not include any of that.

Bonus: Whether the connection is wired or wireless, if one gets connected through VPN to a remote site with a tun device, supposing the tun device opened for that connection is tun0, and the prior iptables rules are applied, then the remote connection gets also shared with the guest. This avoids the need for the guest to also open a VPN connection. Again, as the guest networking needs to be static, then if connecting the host remotely this way, one most probably will need to edit the DNS servers on the guest.

Networking with VDE2

Tango-edit-clear.pngThis article or section needs language, wiki syntax or style improvements.Tango-edit-clear.png

Reason: This section needs serious cleanup and may contain out-of-date information. (Discuss in Talk:QEMU (简体中文)#)

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 we 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. You 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.

Basics

VDE support can be installed via the vde2 package in the official repositories.

In our config, we use tun/tap to create a virtual interface on my host. Load the tun module (see Kernel modules for details):

# modprobe tun

Now create the virtual switch:

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

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

The interface is plugged in but not configured yet. To configure it, run this command:

# ip addr add 192.168.100.254/24 dev tap0

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

$ qemu-system-i386 -net nic -net vde -hda [...]

Configure networking for your guest as you would do in a physical network.

Tip: You might want to set up NAT on tap device to access the internet from the virtual machine. See Internet sharing#Enable NAT for more information.

Startup scripts

Example of main script starting VDE:

/etc/systemd/scripts/qemu-network-env
#!/bin/sh
# QEMU/VDE network environment preparation script

# The IP configuration for the tap device that will be used for
# the virtual machine network:

TAP_DEV=tap0
TAP_IP=192.168.100.254
TAP_MASK=24
TAP_NETWORK=192.168.100.0

# Host interface
NIC=eth0

case "$1" in
  start)
        echo -n "Starting VDE network for QEMU: "

        # If you want tun kernel module to be loaded by script uncomment here
	#modprobe tun 2>/dev/null
	## Wait for the module to be loaded
 	#while ! lsmod | grep -q "^tun"; do echo "Waiting for tun device"; sleep 1; done

        # Start tap switch
        vde_switch -tap "$TAP_DEV" -daemon -mod 660 -group users

        # Bring tap interface up
        ip address add "$TAP_IP"/"$TAP_MASK" dev "$TAP_DEV"
        ip link set "$TAP_DEV" up

        # Start IP Forwarding
        echo "1" > /proc/sys/net/ipv4/ip_forward
        iptables -t nat -A POSTROUTING -s "$TAP_NETWORK"/"$TAP_MASK" -o "$NIC" -j MASQUERADE
        ;;
  stop)
        echo -n "Stopping VDE network for QEMU: "
        # Delete the NAT rules
        iptables -t nat -D POSTROUTING "$TAP_NETWORK"/"$TAP_MASK" -o "$NIC" -j MASQUERADE

        # Bring tap interface down
        ip link set "$TAP_DEV" down

        # Kill VDE switch
        pgrep -f vde_switch | xargs kill -TERM
        ;;
  restart|reload)
        $0 stop
        sleep 1
        $0 start
        ;;
  *)
        echo "Usage: $0 {start|stop|restart|reload}"
        exit 1
esac
exit 0

Example of systemd service using the above script:

/etc/systemd/system/qemu-network-env.service
[Unit]
Description=Manage VDE Switch

[Service]
Type=oneshot
ExecStart=/etc/systemd/scripts/qemu-network-env start
ExecStop=/etc/systemd/scripts/qemu-network-env stop
RemainAfterExit=yes

[Install]
WantedBy=multi-user.target

Change permissions for qemu-network-env to be executable

# chmod u+x /etc/systemd/scripts/qemu-network-env

You can start qemu-network-env.service as usual.

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 users
# slirpvde --dhcp --daemon

Then, to start the VM with a connection to the network of the host:

$ qemu-system-i386 -net nic,macaddr=52:54:00:00:EE:03 -net vde disk_image

VDE2 Bridge

Based on quickhowto: qemu networking using vde, tun/tap, and bridge graphic. Any virtual machine connected to vde is externally exposed. For example, each virtual machine can receive DHCP configuration directly from your ADSL router.

Basics

Remember that you need tun module and bridge-utils package.

Create the vde2/tap device:

# vde_switch -tap tap0 -daemon -mod 660 -group users
# ip link set tap0 up

Create bridge:

# brctl addbr br0

Add devices:

# brctl addif br0 eth0
# brctl addif br0 tap0

And configure bridge interface:

# dhcpcd br0

Startup scripts

All devices must be set up. And only the bridge needs an IP address. For physical devices on the bridge (e.g. eth0), this can be done with netctl using a custom Ethernet profile with:

/etc/netctl/ethernet-noip
Description='A more versatile static Ethernet connection'
Interface=eth0
Connection=ethernet
IP=no

The following custom systemd service can be used to create and activate a VDE2 tap interface for use in the users user group.

/etc/systemd/system/vde2@.service
[Unit]
Description=Network Connectivity for %i
Wants=network.target
Before=network.target

[Service]
Type=oneshot
RemainAfterExit=yes
ExecStart=/usr/bin/vde_switch -tap %i -daemon -mod 660 -group users
ExecStart=/usr/bin/ip link set dev %i up
ExecStop=/usr/bin/ip addr flush dev %i
ExecStop=/usr/bin/ip link set dev %i down

[Install]
WantedBy=multi-user.target

And finally, you can create the bridge interface with netctl.

图形

QEMU 可以使用一下几个图形输出:std, cirrus, vmware, qxl, xenfs 和 vnc。

使用 vnc 选项,你可以单独运行客户机,并且通过 VNC 连接。其他选项是使用std, vmware, cirrus:

std

使用 -vga std 你可以得到最高 2560 x 1600 像素的分辨率。从 QEMU 2.2 开始是默认选项。

qxl

QXL is a paravirtual graphics driver with 2D support. To use it, pass the -vga qxl option and install drivers in the guest. You may want to use SPICE for improved graphical performance when using QXL.

On Linux guests, the qxl and bochs_drm kernel modules must be loaded in order to gain a decent performance.

SPICE

The SPICE project aims to provide a complete open source solution for remote access to virtual machines in a seamless way.

SPICE can only be used when using QXL as the graphical output.

The following is example of booting with SPICE as the remote desktop protocol:

$ qemu-system-i386 -vga qxl -spice port=5930,disable-ticketing -chardev spicevm 

Connect to the guest by using a SPICE client. At the moment spice-gtk3 is recommended, however other clients, including other platforms, are available:

$ spicy -h 127.0.0.1 -p 5930

Using Unix sockets instead of TCP ports does not involve using network stack on the host system, so it is reportedly better for performance. Example:

$ qemu-system-x86_64 -vga qxl -device virtio-serial-pci -device virtserialport,chardev=spicechannel0,name=com.redhat.spice.0 -chardev spicevmc,id=spicechannel0,name=vdagent -spice unix,addr=/tmp/vm_spice.socket,disable-ticketing,playback-compression=off
$ spicy --uri="spice+unix:///tmp/vm_spice.socket"

For improved support for multiple monitors, clipboard sharing, etc. the following packages should be installed on the guest:

vmware

尽管有一点怪怪的,但是这种方法确实比 std 和 cirrus 效果好。在客户机中,安装 软件包 xf86-video-vmwarexf86-input-vmmouse

virtio

virtio-vga / virtio-gpu is a paravirtual 3D graphics driver based on virgl. Currently a work in progress, supporting only very recent (>= 4.4) Linux guests.

cirrus

The cirrus graphical adapter was the default before 2.2. It should not be used on modern systems.

none

This is like a PC that has no VGA card at all. You would not even be able to access it with the -vnc option. Also, this is different from the -nographic option which lets QEMU emulate a VGA card, but disables the SDL display.

vnc

Given that you used the -nographic option, you can add the -vnc display option to have QEMU listen on display and redirect the VGA display to the VNC session. There is an example of this in the #Starting QEMU virtual machines on boot section's example configs.

$ qemu-system-i386 -vga std -nographic -vnc :0
$ gvncviewer :0

When using VNC, you might experience keyboard problems described (in gory details) here. The solution is not to use the -k option on QEMU, and to use gvncviewer from gtk-vnc. See also this message posted on libvirt's mailing list.

Installing virtio drivers

QEMU offers guests the ability to use paravirtualized block and network devices using the virtio drivers, which provide better performance and lower overhead.

  • A virtio block device requires the option -drive instead of the simple -hd* plus if=virtio:
$ qemu-system-i386 -boot order=c -drive file=disk_image,if=virtio
Note: -boot order=c is absolutely necessary when you want to boot from it. There is no auto-detection as with -hd*.
  • Almost the same goes for the network:
$ qemu-system-i386 -net nic,model=virtio
Note: This will only work if the guest machine has drivers for virtio devices. Linux does, and the required drivers are included in Arch Linux, but there is no guarantee that virtio devices will work with other operating systems.

Preparing an (Arch) Linux guest

To use virtio devices after an Arch Linux guest has been installed, the following modules must be loaded in the guest: virtio, virtio_pci, virtio_blk, virtio_net, and virtio_ring. For 32-bit guests, the specific "virtio" module is not necessary.

If you want to boot from a virtio disk, the initial ramdisk must contain the necessary modules. By default, this is handled by mkinitcpio's autodetect hook. Otherwise use the MODULES array in /etc/mkinitcpio.conf to include the necessary modules and rebuild the initial ramdisk.

/etc/mkinitcpio.conf
MODULES="virtio virtio_blk virtio_pci virtio_net"

Virtio disks are recognized with the prefix v (e.g. vda, vdb, etc.); therefore, changes must be made in at least /etc/fstab and /boot/grub/grub.cfg when booting from a virtio disk.

Tip: When referencing disks by UUID in both /etc/fstab and bootloader, nothing has to be done.

Further information on paravirtualization with KVM can be found here.

You might also want to install qemu-guest-agent to implement support for QMP commands that will enhance the hypervisor management capabilities. After installing the package you can enable and start the qemu-ga.service.

Preparing a Windows guest

Note: The only (reliable) way to upgrade a Windows 8.1 guest to Windows 10 seems to be to temporarily choose cpu core2duo,nx for the install [2]. After the install, you may revert to other cpu settings (8/8/2015).

Block device drivers

New Install of Windows

Windows does not come with the virtio drivers. Therefore, you will need to load them during installation. There are basically two ways to do this: via Floppy Disk or via ISO files. Both images can be downloaded from the Fedora repository.

The floppy disk option is difficult because you will need to press F6 (Shift-F6 on newer Windows) at the very beginning of powering on the QEMU. This is difficult since you need time to connect your VNC console window. You can attempt to add a delay to the boot sequence. See man qemu-system for more details about applying a delay at boot.

The ISO option to load drivers is the preferred way, but it is available only on Windows Vista and Windows Server 2008 and later. The procedure is to load the image with virtio drivers in an additional cdrom device along with the primary disk device and Windows installer:

$ qemu-system-i386 ... \
-drive file=/path/to/primary/disk.img,index=0,media=disk,if=virtio \
-drive file=/path/to/installer.iso,index=2,media=cdrom \
-drive file=/path/to/virtio.iso,index=3,media=cdrom \
...

During the installation, the Windows installer will ask you for your Product key and perform some additional checks. When it gets to the "Where do you want to install Windows?" screen, it will give a warning that no disks are found. Follow the example instructions below (based on Windows Server 2012 R2 with Update).

  • Select the option Load Drivers.
  • Uncheck the box for "Hide drivers that aren't compatible with this computer's hardware".
  • Click the Browse button and open the CDROM for the virtio iso, usually named "virtio-win-XX".
  • Now browse to E:\viostor\[your-os]\amd64, select it, and press OK.
  • Click Next

You should now see your virtio disk(s) listed here, ready to be selected, formatted and installed to.

Change Existing Windows VM to use virtio

Modifying an existing Windows guest for booting from virtio disk is a bit tricky.

You can download the virtio disk driver from the Fedora repository.

Now you need to create a new disk image, which fill force Windows to search for the driver. For example:

$ qemu-img create -f qcow2 fake.qcow2 1G

Run the original Windows guest (with the boot disk still in IDE mode) with the fake disk (in virtio mode) and a CD-ROM with the driver.

$ qemu-system-i386 -m 512 -vga std -drive file=windows_disk_image,if=ide -drive file=fake.qcow2,if=virtio -cdrom virtio-win-0.1-81.iso

Windows will detect the fake disk and try to find a driver for it. If it fails, go to the Device Manager, locate the SCSI drive with an exclamation mark icon (should be open), click Update driver and select the virtual CD-ROM. Do not forget to select the checkbox which says to search for directories recursively.

When the installation is successful, you can turn off the virtual machine and launch it again, now with the boot disk attached in virtio mode:

$ qemu-system-i386 -m 512 -vga std -drive file=windows_disk_image,if=virtio
Note: If you encounter the Blue Screen of Death, make sure you did not forget the -m parameter, and that you do not boot with virtio instead of ide for the system drive before drivers are installed.

Network drivers

Installing virtio network drivers is a bit easier, simply add the -net argument as explained above.

$ qemu-system-i386 -m 512 -vga std -drive file=windows_disk_image,if=virtio -net nic,model=virtio -cdrom virtio-win-0.1-74.iso

Windows will detect the network adapter and try to find a driver for it. If it fails, go to the Device Manager, locate the network adapter with an exclamation mark icon (should be open), click Update driver and select the virtual CD-ROM. Do not forget to select the checkbox which says to search for directories recursively.

Preparing a FreeBSD guest

Install the emulators/virtio-kmod port if you are using FreeBSD 8.3 or later up until 10.0-CURRENT where they are included into the kernel. After installation, add the following to your /boot/loader.conf file:

virtio_loader="YES"
virtio_pci_load="YES"
virtio_blk_load="YES"
if_vtnet_load="YES"
virtio_balloon_load="YES"

Then modify your /etc/fstab by doing the following:

sed -i bak "s/ada/vtbd/g" /etc/fstab

And verify that /etc/fstab is consistent. If anything goes wrong, just boot into a rescue CD and copy /etc/fstab.bak back to /etc/fstab.

Tips and tricks

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 systemd unit and config.

/etc/systemd/system/qemu@.service
[Unit]
Description=QEMU virtual machine

[Service]
Environment="type=system-x86_64" "haltcmd=kill -INT $MAINPID"
EnvironmentFile=/etc/conf.d/qemu.d/%i
ExecStart=/usr/bin/env qemu-${type} -name %i -nographic $args
ExecStop=/bin/sh -c ${haltcmd}
TimeoutStopSec=30
KillMode=none

[Install]
WantedBy=multi-user.target
Note: According to systemd.service(5) and systemd.kill(5) man pages it is necessary to use the KillMode=none option. Otherwise the main qemu process will be killed immediately after the ExecStop command quits (it simply echoes one string) and your quest system will not be able to shutdown correctly.

Then create per-VM configuration files, named /etc/conf.d/qemu.d/vm_name, with the following variables set:

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 type="system-arm".
args
QEMU command line to start with. Will always be prepended with -name ${vm} -nographic.
haltcmd
Command to shut down a 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.

Example configs:

/etc/conf.d/qemu.d/one
type="system-x86_64"

args="-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"

haltcmd="echo 'system_powerdown' | nc localhost 7100" # or netcat/ncat

# You can use other ways to shut down your VM correctly
#haltcmd="ssh powermanager@vm1 sudo poweroff"
/etc/conf.d/qemu.d/two
args="-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"

haltcmd="echo 'system_powerdown' | nc localhost 7101"

To set which virtual machines will start on boot-up, enable the qemu@vm_name.service systemd unit.

Mouse integration

To prevent the mouse from being grabbed when clicking on the guest operating system's window, add the option -usbdevice tablet. This means QEMU is able to report the mouse position without having to grab the mouse. This also overrides PS/2 mouse emulation when activated. For example:

$ qemu-system-i386 -hda disk_image -m 512 -vga std -usbdevice tablet

If that does not work, try the tip at #Mouse cursor is jittery or erratic.

Pass-through host USB device

To access physical USB device connected to host from VM, you can start QEMU with following option:

$ qemu-system-i386 -usbdevice host:vendor_id:product_id disk_image

You can find vendor_id and product_id of your device with lsusb command.

Note: If you encounter permission errors when running QEMU, see Udev#Writing udev rules for information on how to set permissions of the device.

Enabling KSM

Kernel Samepage Merging (KSM) is a feature of the Linux kernel that allows for an application to register with the kernel to have its pages merged with other processes that also register to have their pages merged. The KSM mechanism allows for guest virtual machines to share pages with each other. In an environment where many of the guest operating systems are similar, this can result in significant memory savings.

To enable KSM, simply run

# echo 1 > /sys/kernel/mm/ksm/run

To make it permanent, you can use systemd's temporary files:

/etc/tmpfiles.d/ksm.conf
w /sys/kernel/mm/ksm/run - - - - 1

If KSM is running, and there are pages to be merged (i.e. at least two similar VMs are running), then /sys/kernel/mm/ksm/pages_shared should be non-zero. See https://www.kernel.org/doc/Documentation/vm/ksm.txt for more information.

Tip: An easy way to see how well KSM is performing is to simply print the contents of all the files in that directory:
$ grep . /sys/kernel/mm/ksm/*

Multi-monitor support

The Linux QXL driver supports four heads (virtual screens) by default. This can be changed via the qxl.heads=N kernel parameter.

The default VGA memory size for QXL devices is 16M (VRAM size is 64M). This is not sufficient if you would like to enable two 1920x1200 monitors since that requires 2 × 1920 × 4 (color depth) × 1200 = 17.6 MiB VGA memory. This can be changed by replacing -vga qxl by -vga none -device qxl-vga,vgamem_mb=32. If you ever increase vgamem_mb beyond 64M, then you also have to increase the vram_size_mb option.

Copy and paste

To have copy and paste between the host and the guest you need to enable the spice agent communication channel. It requires to add a virtio-serial device to the guest, and open a port for the spice vdagent. It is also required to install the spice vdagent in guest (spice-vdagentAUR for Arch guests, Windows guest tools for Windows guests). Make sure the agent is running (and for future, started automatically).

Start QEMU with the following options:

$ qemu-system-i386 -vga qxl -spice port=5930,disable-ticketing -device virtio-serial-pci -device virtserialport,chardev=spicechannel0,name=com.redhat.spice.0 -chardev spicevmc,id=spicechannel0,name=vdagent

The -device virtio-serial-pci option adds the virtio-serial device, -device virtserialport,chardev=spicechannel0,name=com.redhat.spice.0 opens a port for spice vdagent in that device and -chardev spicevmc,id=spicechannel0,name=vdagent adds a spicevmc chardev for that port.

It is important that the chardev= option of the virtserialport device matches the id= option given to the chardev option (spicechannel0 in this example). It is also important that the port name is com.redhat.spice.0, because that is the namespace where vdagent is looking for in the guest. And finally, specify name=vdagent so that spice knows what this channel is for.

Windows-specific notes

QEMU can run any version of Windows from Windows 95 through Windows 10.

It is possible to run Windows PE in QEMU.

Fast startup

For Windows 8 (or later) guests it is better to disable "Fast Startup" from the Power Options of the Control Panel, as it causes the guest to hang during every other boot.

Fast Startup may also need to be disabled for changes to the -smp option to be properly applied.

Remote Desktop Protocol

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

$ qemu-system-i386 -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

Troubleshooting

Mouse cursor is jittery or erratic

If the cursor jumps around the screen uncontrollably, entering this on the terminal before starting QEMU might help:

$ export SDL_VIDEO_X11_DGAMOUSE=0

If this helps, you can add this to your ~/.bashrc file.

No visible Cursor

Add -show-cursor to QEMU's options to see a mouse cursor.

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-system-i386 -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 the -cpu host option to make QEMU emulate the host's exact CPU. If you do not do this, it may be trying to emulate a more generic CPU.
  • If the host machine has multiple CPUs, assign the guest more CPUs using the -smp option.
  • Make sure you have assigned the virtual machine enough memory. By default, QEMU only assigns 128 MiB of memory to each virtual machine. Use the -m option to assign more memory. For example, -m 1024 runs a virtual machine with 1024 MiB of memory.
  • Use KVM if possible: add -machine type=pc,accel=kvm to the QEMU start command you use.
  • If supported by drivers in the guest operating system, use virtio for network and/or block devices. For example:
$ qemu-system-i386 -net nic,model=virtio -net tap,if=tap0,script=no -drive file=disk_image,media=disk,if=virtio
  • Use TAP devices instead of user-mode networking. See #Tap networking with QEMU.
  • If the guest OS is doing heavy writing to its disk, you may benefit from certain mount options on the host's file system. For example, you can mount an ext4 file system with the option barrier=0. You should read the documentation for any options that you change because sometimes performance-enhancing options for file systems come at the cost of data integrity.
  • If you have a raw disk image, you may want to disable the cache:
$ qemu-system-i386 -drive file=disk_image,if=virtio,cache=none
  • Use the native Linux AIO:
$ qemu-system-i386 -drive file=disk_image,if=virtio,aio=native
  • 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.

See http://www.linux-kvm.org/page/Tuning_KVM for more information.

Guest display stretches on window resize

To restore default window size, press Ctrl+Alt+u.

ioctl(KVM_CREATE_VM) failed: 16 Device or resource busy

If an error message like this is printed when starting QEMU with -enable-kvm option:

ioctl(KVM_CREATE_VM) failed: 16 Device or resource busy
failed to initialize KVM: Device or resource busy

that means another hypervisor is currently running. It is not recommended or possible to run several hypervisors in parallel.

libgfapi error message

The error message displayed at startup:

Failed to open module: libgfapi.so.0: cannot open shared object file: No such file or directory

is not a problem, it just means that you are lacking the optional GlusterFS dependency.

Kernel panic on LIVE-environments

If you start a live-environment (or better: booting a system) you may encounter this:

[ end Kernel panic - not syncing: VFS: Unable to mount root fs on unknown block(0,0)

or some other boot hindering process (e.g. cannot unpack initramfs, cant start service foo). Try starting the VM with the -m VALUE switch and an appropriate amount of RAM, if the ram is to low you will probably encounter similar issues as above/without the memory-switch.

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