KVM

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KVM, Kernel-based Virtual Machine, is a hypervisor built into the Linux kernel. It is similar to Xen in purpose but much simpler to get running. Unlike native QEMU, which uses emulation, KVM is a special operating mode of QEMU that uses CPU extensions (HVM) for virtualization via a kernel module.

Using KVM, one can run multiple virtual machines running unmodified GNU/Linux, Windows, or any other operating system. (See Guest Support Status for more information.) Each virtual machine has private virtualized hardware: a network card, disk, graphics card, etc.

Differences between KVM and Xen, VMware, or QEMU can be found at the KVM FAQ.

This article does not cover features common to multiple emulators using KVM as a backend. You should see related articles for such information.

Checking support for KVM

Hardware support

KVM requires that the virtual machine host's processor has virtualization support (named VT-x for Intel processors and AMD-V for AMD processors). You can check whether your processor supports hardware virtualization with the following command:

$ lscpu

Your processor supports virtualization only if there is a line telling you so.

You can also run:

$ egrep --color=auto 'vmx|svm|0xc0f' /proc/cpuinfo

If nothing is displayed after running that command, then your processor does not support hardware virtualization, and you will not be able to use KVM.

Note: You may need to enable virtualization support in your BIOS.

Kernel support

Arch Linux kernels provide the appropriate kernel modules to support KVM and VIRTIO.

KVM modules

You can check if necessary modules (kvm and one of kvm_amd, kvm_intel) are available in your kernel with the following command (assuming your kernel is built with CONFIG_IKCONFIG_PROC):

$ zgrep CONFIG_KVM /proc/config.gz

If the module is not set equal to y or m, then the module is not available.

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 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)

How to use KVM

See the main article: QEMU.

Tips and tricks

Note: See QEMU#Tips and tricks and QEMU#Troubleshooting for general tips and tricks.

Nested virtualization

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

Reason: Is it possible also with kvm_amd? (Discuss in Talk:KVM#)

Nested virtualization enables existing virtual machines to be run on third-party hypervisors and on other clouds without any modifications to the original virtual machines or their networking.

On host, enable nested feature for kvm_intel:

# modprobe -r kvm_intel
# modprobe kvm_intel nested=1

To make it permanent (see Kernel modules#Setting module options):

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

Verify that feature is activated:

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

Run guest VM with following command:

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

Boot VM and check if vmx flag is present:

$ egrep --color=auto 'vmx|svm' /proc/cpuinfo

Alternative Networking with SSH tunnels

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#)

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#)

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#)

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#)

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

Notes: qemu-kvm 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#)

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.

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:

$ grep Hugepagesize /proc/meminfo

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:

$ grep HugePages_Total /proc/meminfo 
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:

$ grep HugePages /proc/meminfo 
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/40-hugepage.conf:

vm.nr_hugepages = 550

See also:

See also