Kvm guest tools linux

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:

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

Kernel support

Arch Linux kernels provide the required kernel modules to support KVM.

One can check if the necessary modules, kvm and either kvm_amd or kvm_intel , are available in the kernel with the following command:

The module is available only if it is set to either y or m .

Then, ensure that the kernel modules are automatically loaded, with the command:

If the command returns nothing, the module needs to be loaded manually, see Kernel modules#Manual module handling.

Para-virtualization with Virtio

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.

Kernel support

Use the following command to check if the VIRTIO modules are available in the kernel inside the virtual machine:

Then, check if the kernel modules are automatically loaded with the command:

In case the above commands return nothing, you need to load the kernel modules manually.

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

Nested virtualization

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 :

Verify that feature is activated:

Enable the «host passthrough» mode to forward all CPU features to the guest system:

If using QEMU, run the guest virtual machine with the following command: qemu-system-x86_64 -enable-kvm -cpu host .
If using virt-manager, change the CPU model to host-passthrough .
If using virsh, use virsh edit vm-name and change the CPU line to

Boot VM and check if vmx flag is present:

Enabling huge pages

This article or section is a candidate for merging with QEMU.

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. The default permission is root’s uid and gid with 0755, but we want anyone in the kvm group to have access to hugepages.

Add to your /etc/fstab :

Of course the gid must match that of the kvm group or specify the group name directly with gid=kvm . 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:

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

Normally that should be 2048 kB ≙ 2 MB. Let us 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:

If you had enough free memory you should see:

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

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:

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