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.
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
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:
$ LC_ALL=C lscpu | grep Virtualization
$ grep -E --color=auto 'vmx|svm|0xc0f' /proc/cpuinfo
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.
Arch Linux kernels provide the required kernel modules to support KVM.
- One can check if the necessary modules,
kvm_intel, are available in the kernel with the following command:
$ zgrep CONFIG_KVM /proc/config.gz
The module is available only if it is set to either
- Then, ensure that the kernel modules are automatically loaded, with the command:
$ lsmod | grep kvm
kvm_intel 245760 0 kvmgt 28672 0 mdev 20480 2 kvmgt,vfio_mdev vfio 32768 3 kvmgt,vfio_mdev,vfio_iommu_type1 kvm 737280 2 kvmgt,kvm_intel irqbypass 16384 1 kvm
If the command returns nothing, the module needs to be loaded manually, see Kernel modules#Manual module handling.
kvm_amdfails but modprobing
lscpuclaims that hardware acceleration is supported, check the BIOS settings. Some vendors, especially laptop vendors, disable these processor extensions by default. To determine whether there is no hardware support or whether the extensions are disabled in BIOS, the output from
dmesgafter having failed to modprobe will tell.
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.
Use the following command to check if the VIRTIO modules are available in the kernel inside the virtual machine:
$ zgrep VIRTIO /proc/config.gz
Then, check if the kernel modules are automatically loaded with the command:
$ lsmod | grep virtio
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 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
# modprobe -r kvm_intel # modprobe kvm_intel nested=1
To make it permanent (see Kernel modules#Setting module options):
options kvm_intel nested=1
Verify that feature is activated:
$ systool -m kvm_intel -v | grep nested
nested = "Y"
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(it will not be in the list, just write it in the box).
- If using virsh, use
virsh edit vm-nameand change the CPU line to
<cpu mode='host-passthrough' check='partial'/>
Boot VM and check if vmx flag is present:
$ grep -E --color=auto 'vmx|svm' /proc/cpuinfo
Enabling huge pages
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
hugetlbfs /dev/hugepages hugetlbfs mode=01770,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 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:
# echo 550 > /proc/sys/vm/nr_hugepages
If you had enough free memory you should see:
$ grep HugePages_Total /proc/meminfo
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> [...]
-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
vm.nr_hugepages = 550