KVM (正體中文)

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註記: please use the first argument of the template to provide more detailed indications. (討論)

KVM(Kernel-based Virtual Machine的縮寫)是於Linux內核內建的虛擬機。有點類似於 Xen ,但更追求更簡單的運作,比如執行此虛擬機器,只需要載入相應的 kvm 模組即可後台待命。和 Xen 的半虛擬不同的是,KVM 需要電腦支援虛擬化技術(Intel的 VT 扩展或者 AMD 的 AMD-V 擴充)。Xen的全虛擬化仍然要求電腦支援虛擬化技術。

在KVM中,可以執行各種未更改的GNU/Linux, Windows 或任何其他系統鏡像。(請看[1]),每個虛擬機器都可提供獨享的虛擬硬體:網路卡,硬碟,顯卡等。请看 KVM Howto

KVM, Xen, VMware, 和 QEMU 的不同之處,請看此處KVM FAQ.

檢查您的電腦是否支援KVM

硬體支援

KVM需要宿主(host)機的處理器支援虛擬化(對於Intel CPU來說是VT-x,對於AMD CPU來說是AMD-V)。你可以通過lscpu命令來檢查你的CPU是否支援虛擬化:

$ lscpu

如果你的CPU支援虛擬化,輸出結果中會有一行Virtualization的諮詢。

你也可以執行:

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

如果執行後沒有結果,那麼你的CPU支援硬體虛擬化,你不能使用KVM。

注意: 您可能需要在BIOS中啟用虛擬化支援

內核支援

Arch Linux的內核提供了相應的內核模組來支援KVM和VirtIO。

KVM模組

如果你的內核是用 CONFIG_IKCONFIG_PROC 這個選項編譯的話,你可以通過以下命令檢查你的內核是否已經包含了支援KVM所必須的模組(kvmkvm_amdkvm_intel這両者中的任意一個):

$ zgrep KVM /proc/config.gz

如果模組設定不等於 ym,則該模組可用

Para-virtualized devices

Para-virtualization provides a fast and efficient means of communication for guests to use devices on the host machine. KVM provides para-virtualized devices to virtual machines using the Virtio API as a layer between the hypervisor and guest.

All virtio devices have two parts: the host device and the guest driver.

VirtIO modules

Use the following command to check if needed modules are available:

$ zgrep VIRTIO /proc/config.gz

Loading kernel modules

First, check if the kernel modules are automatically loaded. This should be the case with recent versions of udev.

$ lsmod | grep kvm
$ lsmod | grep virtio

In case the above commands return nothing, you need to load[broken link: invalid section] kernel modules.

Tip: If modprobing kvm_intel or kvm_amd fails but modprobing kvm succeeds, (and lscpu claims that hardware acceleration is supported), check your BIOS settings. Some vendors (especially laptop vendors) disable these processor extensions by default. To determine whether there's no hardware support or there is but the extensions are disabled in BIOS, the output from dmesg after having failed to modprobe will tell.

List of para-virtualized devices

  • network device (virtio-net)
  • block device (virtio-blk)
  • controller device (virtio-scsi)
  • serial device (virtio-serial)
  • balloon device (virtio-balloon)

如何使用KVM

必須使用QEMULibvirt作為KVM的控制界面和前端,請參考Libvirt (正體中文)條目和QEMU條目。

小貼士与小技巧

注意: 請參考QEMU#Tips and tricksQEMU#Troubleshooting詞條。

嵌套虛擬化

Tango-view-fullscreen.pngThis article or section needs expansion.Tango-view-fullscreen.png

Reason: 這是否可以被用在kvm_amd模組上? (Discuss in Talk:KVM (正體中文)#)

在宿主機啟用kvm_intel模組的嵌套虛擬化機能:

# modprobe -r kvm_intel
# modprobe kvm_intel nested=1

使嵌套虛擬化永久生效(請參考Kernel modules#Setting module options):

/etc/modprobe.d/modprobe.conf
options kvm_intel nested=1

檢驗嵌套虛擬化功能是否已被激活:

$ systool -m kvm_intel -v | grep nested
    nested              = "Y"

使用以下命令來執行guest虛擬機器:

$ qemu-system-x86_64 -enable-kvm -cpu host

啟動虛擬機並檢查是否有vmx標誌:

$ grep vmx /proc/cpuinfo

Live snapshots

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

Notes: virsh is part of libvirt (Discuss in Talk:KVM (正體中文)#)

A feature called external snapshotting allows one to take a live snapshot of a virtual machine without turning it off. Currently it only works with qcow2 and raw file based images.

Once a snapshot is created, KVM attaches that new snapshotted image to virtual machine that is used as its new block device, storing any new data directly to it while the original disk image is taken offline which you can easily copy or backup. After that you can merge the snapshotted image to the original image, again without shutting down your virtual machine.

Here's how it works.

Current running vm

# virsh list --all
Id    Name                           State
----------------------------------------------------
3     archey                            running

List all its current images

# virsh domblklist archey 
Target     Source
------------------------------------------------
vda        /vms/archey.img

Notice the image file properties

# qemu-img info /vms/archey.img
image: /vms/archey.img
file format: qcow2
virtual size: 50G (53687091200 bytes)
disk size: 2.1G
cluster_size: 65536

Create a disk-only snapshot. The switch --atomic makes sure that the VM is not modified if snapshot creation fails.

# virsh snapshot-create-as archey snapshot1 --disk-only --atomic

List if you want to see the snapshots

# virsh snapshot-list archey
Name                 Creation Time             State
------------------------------------------------------------
snapshot1           2012-10-21 17:12:57 -0700 disk-snapshot

Notice the new snapshot image created by virsh and its image properties. It weighs just a few MiBs and is linked to its original "backing image/chain".

# qemu-img info /vms/archey.snapshot1
image: /vms/archey.snapshot1
file format: qcow2
virtual size: 50G (53687091200 bytes)
disk size: 18M
cluster_size: 65536
backing file: /vms/archey.img

At this point, you can go ahead and copy the original image with cp -sparse=true or rsync -S. Then you can merge the original image back into the snapshot.

# virsh blockpull --domain archey --path /vms/archey.snapshot1

Now that you have pulled the blocks out of original image, the file /vms/archey.snapshot1 becomes the new disk image. Check its disk size to see what it means. After that is done, the original image /vms/archey.img and the snapshot metadata can be deleted safely. The virsh blockcommit would work opposite to blockpull but it seems to be currently under development in qemu-kvm 1.3 (including snapshot-revert feature), scheduled to be released sometime next year.

This new feature of KVM will certainly come handy to the people who like to take frequent live backups without risking corruption of the file system.

Poor Man's Networking

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

Notes: This section is not KVM-specific, it's generally applicable to all QEMU VMs. (Discuss in Talk:KVM (正體中文)#)

Setting up bridged networking can be a bit of a hassle sometimes. If the sole purpose of the VM is experimentation, one strategy to connect the host and the guests is to use SSH tunneling.

The basic steps are as follows:

  • Setup an SSH server in the host OS
  • (optional) Create a designated user used for the tunneling (e.g. tunneluser)
  • Install SSH in the VM
  • Setup authentication

See: SSH for the setup of SSH, especially SSH#Forwarding other ports.

When using the default user network stack, the host is reachable at address 10.0.2.2.

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

Reason: Usage of /etc/rc.local is discouraged. This should be a proper systemd service file. (Discuss in Talk:KVM (正體中文)#)

If everything works and you can SSH into the host, simply add something like the following to your /etc/rc.local

# Local SSH Server
echo "Starting SSH tunnel"
sudo -u vmuser ssh tunneluser@10.0.2.2 -N -R 2213:127.0.0.1:22 -f
# Random remote port (e.g. from another VM)
echo "Starting random tunnel"
sudo -u vmuser ssh tunneluser@10.0.2.2 -N -L 2345:127.0.0.1:2345 -f

In this example a tunnel is created to the SSH server of the VM and an arbitrary port of the host is pulled into the VM.

This is a quite basic strategy to do networking with VMs. However, it is very robust and should be quite sufficient most of the time.

Tango-inaccurate.pngThe factual accuracy of this article or section is disputed.Tango-inaccurate.png

Reason: Isn't this option enough? I think it should have the same effect: -redir tcp:2222:10.0.2.15:22 (it redirects port 2222 from host to 10.0.2.15:22, where 10.0.2.15 is guest's IP address. (Discuss in Talk:KVM (正體中文)#)

Enabling huge pages

Tango-inaccurate.pngThe factual accuracy of this article or section is disputed.Tango-inaccurate.png

Reason: With systemd, hugetlbfs is mounted on /dev/hugepages by default, but with mode 0755 and root's uid and gid. (Discuss in Talk:KVM (正體中文)#)

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

Notes: qemu-kvm[broken link: replaced by qemu] no longer exists as all of its features have been merged into qemu. After the above issue is cleared, I suggest merging this section into QEMU. (Discuss in Talk:KVM (正體中文)#)

You may also want to enable hugepages to improve the performance of your virtual machine. With an up to date Arch Linux and a running KVM you probably already have everything you need. Check if you have the directory /dev/hugepages. If not create it. Now we need the right permissions to use this directory. Check if the group kvm exist and if you are member of this group. This should be the case if you already have a running virtual machine.

$ getent group kvm
kvm:x:78:USERNAMES

Add to your /etc/fstab:

hugetlbfs       /dev/hugepages  hugetlbfs       mode=1770,gid=78        0 0

Of course the gid must match that of the kvm group. The mode of 1770 allows anyone in the group to create files but not unlink or rename each other's files. Make sure /dev/hugepages is mounted properly:

# umount /dev/hugepages
# mount /dev/hugepages
$ mount | grep huge
hugetlbfs on /dev/hugepages type hugetlbfs (rw,relatime,mode=1770,gid=78)

Now you can calculate how many hugepages you need. Check how large your hugepages are:

$ cat /proc/meminfo | grep Hugepagesize

Normally that should be 2048 kB ≙ 2 MB. Let's say you want to run your virtual machine with 1024 MB. 1024 / 2 = 512. Add a few extra so we can round this up to 550. Now tell your machine how many hugepages you want:

# echo 550 > /proc/sys/vm/nr_hugepages

If you had enough free memory you should see:

$ cat /proc/meminfo | grep HugePages_Total
HugesPages_Total:  550

If the number is smaller, close some applications or start your virtual machine with less memory (number_of_pages x 2):

$ qemu-system-x86_64 -enable-kvm -m 1024 -mem-path /dev/hugepages -hda <disk_image> [...]

Note the -mem-path parameter. This will make use of the hugepages.

Now you can check, while your virtual machine is running, how many pages are used:

$ cat /proc/meminfo | grep HugePages
HugePages_Total:     550
HugePages_Free:       48
HugePages_Rsvd:        6
HugePages_Surp:        0

Now that everything seems to work you can enable hugepages by default if you like. Add to your /etc/sysctl.d/80-hugepages.conf:

vm.nr_hugepages = 550

See also:

See also