libvirt

The libvirt library is used to interface with different virtualization technologies. Before getting started with libvirt it is best to make sure your hardware supports the necessary virtualization extensions for KVM. Enter the following from a terminal prompt:

kvm-ok

A message will be printed informing you if your CPU does or does not support hardware virtualization.

Note

On many computers with processors supporting hardware assisted virtualization, it is necessary to activate an option in the BIOS to enable it.

Virtual Networking

There are a few different ways to allow a virtual machine access to the external network. The default virtual network configuration includes bridging and iptables rules implementing usermode networking, which uses the SLIRP protocol. Traffic is NATed through the host interface to the outside network.

To enable external hosts to directly access services on virtual machines a different type of bridge than the default needs to be configured. This allows the virtual interfaces to connect to the outside network through the physical interface, making them appear as normal hosts to the rest of the network.

There is a great example how to configure an own bridge and combining it with libvirt so that guests will use it at the netplan.io.

Installation

To install the necessary packages, from a terminal prompt enter:

sudo apt install qemu-kvm libvirt-daemon-system

After installing libvirt-daemon-system, the user used to manage virtual machines will need to be added to the libvirt group. This is done automatically for members of the sudo group, but needs to be done in additon for anyone else that should access system wide libvirt resources. Doing so will grant the user access to the advanced networking options.

In a terminal enter:

sudo adduser $USER libvirt

Note

If the user chosen is the current user, you will need to log out and back in for the new group membership to take effect.

You are now ready to install a Guest operating system. Installing a virtual machine follows the same process as installing the operating system directly on the hardware.

You either need:

  • a way to automate the installation
  • a keyboard and monitor will need to be attached to the physical machine.
  • use cloud images which are meant to self-initialize (see Multipass and UVTool)

In the case of virtual machines a Graphical User Interface (GUI) is analogous to using a physical keyboard and mouse on a real computer. Instead of installing a GUI the virt-viewer or virt-manager application can be used to connect to a virtual machine’s console using VNC. See Virtual Machine Manager / Viewer for more information.

Virtual Machine Management

The following section covers the command-line tools around virsh that are part of libvirt itself. But there are various options at different levels of complexities and feature-sets, like:

virsh

There are several utilities available to manage virtual machines and libvirt. The virsh utility can be used from the command line. Some examples:

  • To list running virtual machines:

    virsh list
    
  • To start a virtual machine:

    virsh start web_devel
    
  • Similarly, to start a virtual machine at boot:

    virsh autostart web_devel
    
  • Reboot a virtual machine with:

    virsh reboot web_devel
    
  • The state of virtual machines can be saved to a file in order to be restored later. The following will save the virtual machine state into a file named according to the date:

    virsh save web_devel web_devel-022708.state
    

    Once saved the virtual machine will no longer be running.

  • A saved virtual machine can be restored using:

    virsh restore web_devel-022708.state
    
  • To shutdown a virtual machine do:

    virsh shutdown web_devel
    
  • A CDROM device can be mounted in a virtual machine by entering:

    virsh attach-disk web_devel /dev/cdrom /media/cdrom
    

Note

In the above examples replace web_devel with the appropriate virtual machine name, and web_devel-022708.state with a descriptive file name.

If virsh (or other vir* tools) shall connect to something else than the default qemu-kvm/system hipervisor one can find alternatives for the connect option in man virsh or libvirt doc

system and session scope

Virsh - as well as most other tools to manage virtualization - can be passed connection strings.

$ virsh --connect qemu:///system

There are two options for the connection.

  • qemu:///system - connect locally as root to the daemon supervising QEMU and KVM domains
  • qemu:///session - connect locally as a normal user to his own set of QEMU and KVM domains

The default always was (and still is) qemu:///system as that is the behavior users are used to.
But there are a few benefits (and drawbacks) to qemu:///session to consider it.

qemu:///session is per user and can on a multi-user system be used to separate the people.
But most importantly things run under the permissions of the user which means no permission struggle on the just donwloaded image in your $HOME or the just attached USB-stick.
On the other hand it can’t access system resources that well, which includes network setup that is known to be hard with qemu:///session. It falls back to slirp networking which is functional but slow and makes it impossible to be reached from other systems.

qemu:///system is different in that it is run by the global system wide libvirt that can arbitrate resources as needed.
But you might need to mv and/or chown files to the right places permssions to have them usable.

Applications usually will decide on their primary use-case. Desktop-centric applications often choose qemu:///session while most solutions that involve an administrator anyway continue to default to qemu:///system.

Read more about that in the libvirt FAQ and this blog about the topic.

migration

There are different types of migration available depending on the versions of libvirt and the hipervisor being used. In general those types are:

There are various options to those methods, but the entry point for all of them is virsh migrate. Read the integrated help for more detail.

 virsh migrate --help 

Some useful documentation on constraints and considerations about live migration can be found at the Ubuntu Wiki

Device Passthrough / Hotplug

If instead of the here described hotplugging you want to always pass through a device add the xml content of the device to your static guest xml representation via e.g. virsh edit <guestname>. In that case you don’t need to use attach/detach. There are different kinds of passthrough. Types available to you depend on your Hardware and software setup.

  • USB hotplug/passthrough

  • VF hotplug/Passthrough

But both kinds are handled in a very similar way and while there are various way to do it (e.g. also via qemu monitor) driving such a change via libvirt is recommended. That way libvirt can try to manage all sorts of special cases for you and also somewhat masks version differences.

In general when driving hotplug via libvirt you create a xml snippet that describes the device just as you would do in a static guest description. A usb device is usually identified by Vendor/Product id’s:

<hostdev mode='subsystem' type='usb' managed='yes'>
  <source>
    <vendor id='0x0b6d'/>
    <product id='0x3880'/>
  </source>
</hostdev>

Virtual functions are usually assigned via their PCI-ID (domain, bus, slot, function).

<hostdev mode='subsystem' type='pci' managed='yes'>
  <source>
    <address domain='0x0000' bus='0x04' slot='0x10' function='0x0'/>
  </source>
</hostdev>

Note

To get the Virtual function in the first place is very device dependent and can therefore not be fully covered here. But in general it involves setting up an iommu, registering via VFIO and sometimes requesting a number of VFs. Here an example on ppc64el to get 4 VFs on a device:

$ sudo modprobe vfio-pci
# identify device
$ lspci -n -s 0005:01:01.3
0005:01:01.3 0200: 10df:e228 (rev 10)
# register and request VFs
$ echo 10df e228 | sudo tee /sys/bus/pci/drivers/vfio-pci/new_id
$ echo 4 | sudo tee /sys/bus/pci/devices/0005\:01\:00.0/sriov_numvfs
              

You then attach or detach the device via libvirt by relating the guest with the xml snippet.

virsh attach-device <guestname> <device-xml>
# Use the Device int the Guest
virsh detach-device <guestname> <device-xml>

Access Qemu Monitor via libvirt

The Qemu Monitor is the way to interact with qemu/KVM while a guest is running. This interface has many and very powerful features for experienced users. When running under libvirt that monitor interface is bound by libvirt itself for management purposes, but a user can run qemu monitor commands via libvirt still. The general syntax is virsh qemu-monitor-command [options] [guest] 'command'

Libvirt covers most use cases needed, but if you every want/need to work around libvirt or want to tweak very special options you can e.g. add a device that way:

virsh qemu-monitor-command --hmp zesty-test-log 'drive_add 0 if=none,file=/var/lib/libvirt/images/test.img,format=raw,id=disk1'

But since the monitor is so powerful, you can do a lot especially for debugging purposes like showing the guest registers:

virsh qemu-monitor-command --hmp y-ipns 'info registers'
RAX=00ffffc000000000 RBX=ffff8f0f5d5c7e48 RCX=0000000000000000 RDX=ffffea00007571c0
RSI=0000000000000000 RDI=ffff8f0fdd5c7e48 RBP=ffff8f0f5d5c7e18 RSP=ffff8f0f5d5c7df8
[...]

Huge Pages

Using huge pages can help to reduce TLB pressure, page table overhead and speed up some further memory relate actions. Furthermore by default Transparent huge pages are useful, but can be quite some overhead - so if it is clear that using huge pages is preferred making them explicit usually has some gains.

While huge page are admittedly harder to manage, especially later in the lifetime of a system if memory is fragmented they provide a useful boost especially for rather large guests.

Bonus: When using device pass through on very large guests there is an extra benefit of using huge pages as it is faster to do the initial memory clear on vfio dma pin.

Huge page allocation

Huge pages come in different sizes. A normal page usually is 4k and huge pages are eithe 2M or 1G, but depending on the architecture other options are possible.

The most simple, yet least reliable way to allocate some huge pages is to just echo a value to sysfs
Be sure to re-check if it worked.

$ echo 256 | sudo tee /sys/kernel/mm/hugepages/hugepages-2048kB/nr_hugepages
$ cat /sys/kernel/mm/hugepages/hugepages-2048kB/nr_hugepages
256

There one of these sizes is “default huge page size” which will be used in the auto-mounted /dev/hugepages.
Changing the default size requires a reboot and is set via default_hugepagesz

You can check the current default size:

$ grep Hugepagesize /proc/meminfo
Hugepagesize:       2048 kB

But there can be more than one at the same time one better check:

$ tail /sys/kernel/mm/hugepages/hugepages-*/nr_hugepages` 
==> /sys/kernel/mm/hugepages/hugepages-1048576kB/nr_hugepages <==
0
==> /sys/kernel/mm/hugepages/hugepages-2048kB/nr_hugepages <==
2

And even that could on bigger systems be further split per Numa node.

One can allocate huge pages at boot or runtime, but due to fragmentation there are no guarantees it works later. The kernel documentation lists details on both ways.

Huge pages need to be allocated by the kernel as mentioned above but to be consumable they also have to be mounted.
By default systemd will make /dev/hugepages available for the default huge page size.
Feel free to add more mount points if you need different sized.
An overview can be queried with

$ hugeadm --list-all-mounts
Mount Point          Options
/dev/hugepages       rw,relatime,pagesize=2M

A one-stop info for the overall huge page status of the system can be reported with

$ hugeadm --explain

Huge page usage in libvirt

With the above in place libvirt can map guest memory to huge pages.
In a guest definition add the most simple form of

  <memoryBacking>
    <hugepages/>
  </memoryBacking>

That will allocate the huge pages using the default huge page size from a autodetected mountpoint.
For more control e.g. how memory is spread over Numa nodes or which page size to use check out the details at the libvirt doc.

Resources

  • See the KVM home page for more details.

  • For more information on libvirt see the libvirt home page

    • xml configuration of domains and storage being the most often used libvirt reference
  • Another good resource is the Ubuntu Wiki KVM page.

  • For basics how to assign VT-d devices to qemu/KVM, please see the linux-kvm page.

Last updated 17 days ago. Help improve this document in the forum.