Let’s look into the steps to enable nested virtualization on CentOS. Bobcares, as a part of our Server Management Services, offers solutions to every query that comes our way.

How to enable nested virtualization on CentOS?

Nested virtualization refers to setting up a VM’s environment. In other words, we can say that it is a hypervisor feature that helps us to install & run a VM inside a virtual server.

On CentOS, it is disabled by default. To check whether it is active on the host OS, we can use the below code:

hostOS# cat /sys/module/kvm_intel/parameters/nested

If the result is “Y”, then it is active on the host OS. And if the result is “N”, it is not active on the host OS.

Steps to enable:

• Firstly, create a file with the name “/etc/modprobe.d/kvm-nested.conf” with the below content.

  [root@kvm-hypervisor ~]# vi /etc/modprobe.d/kvm-nested.conf
  options kvm-intel nested=1
  options kvm-intel enable_shadow_vmcs=1
  options kvm-intel enable_apicv=1
  options kvm-intel ept=1

   
• Now save the file and exit.
• Remove the ‘kvm_intel module now. Then add it again using the modprobe code. Confirm all VMs are shut down before removing the module so that we can avoid an error.

  [root@kvm-hypervisor ~]# modprobe -r kvm_intel
  [root@kvm-hypervisor ~]# modprobe -a kvm_intel
  [root@kvm-hypervisor ~]#

   
• Then we can check whether the feature is active or not by using the below code:

  [root@kvm-hypervisor ~]# cat /sys/module/kvm_intel/parameters/nested
  Y
  [root@kvm-hypervisor ~]#

Testing

Consider we have a VM with the name “director” on the KVM hypervisor. We have enabled the nested virtualization on it. For the testing, we have to run the below steps:

1. To verify this, we have to make a VM inside a VM. In order to do so, we must make sure the mode of the CPU in the VM is either “host-model” or “host-passthrough”.

2. Run lscpu and lsmod codes after logging into the VM.

[root@kvm-hypervisor ~]# ssh 192.168.126.1 -l root
root@192.168.126.1's password:
Last login: Sun Dec 10 07:05:59 2017 from 192.168.126.254
[root@director ~]# lsmod | grep kvm
kvm_intel             170200  0
kvm                   566604  1 kvm_intel
irqbypass              13503  1 kvm
[root@director ~]#
[root@director ~]# lscpu

3. Now build a VM. We can use the GUI or the virt-install code inside the VM. Here, we are using virt-install code.

[root@director ~]# virt-install  -n Nested-VM  --description "Test Nested VM" \
 --os-type=Linux  --os-variant=rhel7  --ram=2048  --vcpus=2 --disk \
 path=/var/lib/libvirt/images/nestedvm.img,bus=virtio,size=10 --graphics \
 none --location /var/lib/libvirt/images/CentOS-7-x86_64-DVD-1511.iso\
 --extra-args console=ttyS0
Starting install...
Retrieving file .treeinfo...                            | 1.1 kB  00:00:00
Retrieving file vmlinuz...                              | 4.9 MB  00:00:00
Retrieving file initrd.img...                           |  37 MB  00:00:00
Allocating 'nestedvm.img'                               |  10 GB  00:00:00
Connected to domain Nested-VM
Escape character is ^]
[    0.000000] Initializing cgroup subsys cpuset
[    0.000000] Initializing cgroup subsys cpu
[    0.000000] Initializing cgroup subsys cpuacct
[    0.000000] Linux version 3.10.0-327.el7.x86_64 
………………………………………………

We can now create a VM inside another VM. This shows that nested virtualization is all active now.

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Conclusion

Nested Virtualization is not active on CentOS systems by default. In this article, we provide the steps from our Support team to enable it on CentOS.