---
myst:
  html_meta:
    description: Enable nested virtualization on Ubuntu x86 systems using kvm_intel or kvm_amd kernel modules to run VMs inside VMs.
---

(enable-nested-virtualisation)=
# How to enable nested virtualization

The nested virtualization capability allows a guest virtual machine (also referred to as `level 1` or `L1`) that runs on a physical host (`level 0` or `L0`) acting as a hypervisor to create its own guest virtual machines (`L2`).


```{important}
Nested virtualization is enabled by default on Ubuntu. If you are using Ubuntu, it's unlikely that you will need to manually enable the feature. If you check (using the steps below) and discover that nested virtualization is enabled, then you will not need to do anything further.
```

There may be use cases where you need to enable nested virtualization so that you can deploy VMs inside other VMs. The sections below explain how to check if nested virtualization is enabled/available and how to enable it if that is not the case. Bear in mind that currently nested virtualization is only supported in Ubuntu on `x86` machine architecture.


## Check if nested virtualization is enabled

Check if the required kernel module for your CPU is already loaded. Hosts with Intel CPUs require the `kvm_intel` module, while {term}`AMD` hosts require `kvm_amd` instead. Check this with:

```bash
lsmod | grep -i kvm
```

Which should show:

```text
kvm_intel               204800  0
kvm                  1347584  1 kvm_intel
```


### If the module is loaded

If the module is already loaded, you can check if nested virtualization is enabled by running the following command:

```bash
cat /sys/module/kvm_intel/parameters/nested
```

Or, for AMD hosts:

```bash
cat /sys/module/kvm_amd/parameters/nested
```

If the output is either `1` or `Y`, then nested virtualization is enabled and you will not need to manually enable the feature (this should be the case for Ubuntu users).


### If the module is not loaded

If the module your host requires is not loaded, you can load it using `modprobe` in combination with the property `nested=1` to enable nested virtualization, as shown below for Intel hosts:

```bash
modprobe kvm-intel nested=1
```

Or as follows for AMD hosts: 

```bash
modprobe kvm-amd nested=1
```


## Enable nested virtualization 

If the above checks indicate that nested virtualization is not enabled, you can follow the below steps to enable it persistently on the system.

First, create a file in `/etc/modprobe.d` (e.g., `/etc/modprobe.d/kvm.conf`) and add the line `options kvm-intel nested=1` to that file (replace `kvm-intel` with `kvm-amd` for AMD hosts).

Next, reload the kernel module to apply the changes:

```bash
sudo modprobe -r <module>
sudo modprobe <module>
```

```{note}
Make sure to stop all running VMs that might use the kernel module and prevent its reload.
```

Example for Intel hosts:

```bash
sudo modprobe -r kvm-intel
sudo modprobe kvm-intel
```

You should now be able to see nested virtualization enabled (example for Intel hosts):

```bash
cat /sys/module/kvm_intel/parameters/nested
```

If the output is `Y` or `1` then it has been enabled correctly.


## Check and enable nested virtualization inside a VM

Once the nested virtualization is enabled on the host, we can run a `L1` VM
and a `L2` VM inside it. The following instructions assume that you are using libvirt to
manage the VMs but other tools like `multipass` should also work out of the box.

Log into the `L1` VM and check if this can host another `L2` VM on top by running:

```bash
egrep "svm|vmx" /proc/cpuinfo
``` 

If any of these are present in the output (depending on whether the host is AMD or Intel respectively), then virtualization is available in that instance. If this is not the case, you need to edit the instance CPU settings:

* Shut down the instance
* Edit the instance XML definition file by executing: `virsh edit <instance>`
* Search for the `cpu mode` parameter and set its value to either `host-model` or `host-passthrough` (details about these modes can be found [on the OpenStack wiki](https://wiki.openstack.org/wiki/LibvirtXMLCPUModel)).

  Sample `cpu mode` parameter in XML with nested virtualization:

  ```xml
    <cpu mode='host-model' check='partial'/>
  ```
  
* Save the modifications and start the instance


## Limitations of nested virtualization

Nested virtualization has some key limitations you'd need to consider, namely that not all KVM features will be available for instances running nested VMs, and actions such as migrating or saving the parent instance will not be possible until the nested instance is stopped.