How to Enable Nested KVM Settings on AlmaLinux

Introduction

As virtualization gains momentum in modern IT environments, Kernel-based Virtual Machine (KVM) is a go-to choice for developers and administrators managing virtualized systems. AlmaLinux, a robust CentOS alternative, provides an ideal environment for setting up and configuring KVM. One powerful feature of KVM is nested virtualization, which allows you to run virtual machines (VMs) inside other VMs—a feature vital for testing, sandboxing, or multi-layered development environments.

In this guide, we will explore how to enable nested KVM settings on AlmaLinux. We’ll cover prerequisites, step-by-step instructions, and troubleshooting tips to ensure a smooth configuration.

What is Nested Virtualization?

Nested virtualization enables a VM to act as a hypervisor, running other VMs within it. This setup is commonly used for:

• Testing hypervisor configurations without needing physical hardware.
• Training and development, where multiple VM environments simulate real-world scenarios.
• Software development and CI/CD pipelines that involve multiple virtual environments.

KVM’s nested feature is hardware-dependent, requiring specific CPU support for virtualization extensions like Intel VT-x or AMD-V.

Prerequisites

Before diving into the configuration, ensure the following requirements are met:

1. Hardware Support:

   • A processor with hardware virtualization extensions (Intel VT-x or AMD-V).
   • Nested virtualization capability enabled in the BIOS/UEFI.

2. Operating System:

   • AlmaLinux 8 or newer.
   • The latest kernel version for better compatibility.

3. Packages:

   • KVM modules installed (kvm and qemu-kvm).
   • Virtualization management tools (virt-manager, libvirt).

4. Permissions:

   • Administrative privileges to edit kernel modules and configurations.

Step-by-Step Guide to Enable Nested KVM on AlmaLinux

Step 1: Verify Virtualization Support

Confirm your processor supports virtualization and nested capabilities:

grep -E "vmx|svm" /proc/cpuinfo

• Output Explanation:
  • vmx: Indicates Intel VT-x support.
  • svm: Indicates AMD-V support.

If neither appears, check your BIOS/UEFI settings to enable hardware virtualization.

Step 2: Install Required Packages

Ensure you have the necessary virtualization tools:

sudo dnf install qemu-kvm libvirt virt-manager -y

• qemu-kvm: Provides the KVM hypervisor.
• libvirt: Manages virtual machines.
• virt-manager: Offers a graphical interface to manage VMs.

Enable and start the libvirtd service:

sudo systemctl enable --now libvirtd

Step 3: Check and Load KVM Modules

Verify that the KVM modules are loaded:

lsmod | grep kvm

• kvm_intel or kvm_amd should be listed, depending on your processor type.

If not, load the appropriate module:

sudo modprobe kvm_intel    # For Intel processors
sudo modprobe kvm_amd      # For AMD processors

Step 4: Enable Nested Virtualization

Edit the KVM module options to enable nested support.

For Intel processors:

sudo echo "options kvm_intel nested=1" > /etc/modprobe.d/kvm_intel.conf

For AMD processors:

sudo echo "options kvm_amd nested=1" > /etc/modprobe.d/kvm_amd.conf

Update the module settings:

sudo modprobe -r kvm_intel
sudo modprobe kvm_intel

(Replace kvm_intel with kvm_amd for AMD CPUs.)

Step 5: Verify Nested Virtualization

Check if nested virtualization is enabled:

cat /sys/module/kvm_intel/parameters/nested    # For Intel
cat /sys/module/kvm_amd/parameters/nested     # For AMD

If the output is Y, nested virtualization is enabled.

Step 6: Configure Guest VMs for Nested Virtualization

To use nested virtualization, create or modify your guest VM configuration. Using virt-manager:

1. Open the VM settings in virt-manager.
2. Navigate to Processor settings.
3. Enable Copy host CPU configuration.
4. Ensure that virtualization extensions are visible to the guest.

Alternatively, update the VM’s XML configuration:

sudo virsh edit <vm-name>

Add the following to the <cpu> section:

<cpu mode='host-passthrough'/>

Restart the VM for the changes to take effect.

Troubleshooting Tips

1. KVM Modules Fail to Load:

   • Ensure that virtualization is enabled in the BIOS/UEFI.
   • Verify hardware compatibility for nested virtualization.

2. Nested Feature Not Enabled:

   • Double-check /etc/modprobe.d/ configuration files for syntax errors.
   • Reload the kernel modules.

3. Performance Issues:

   • Nested virtualization incurs overhead; ensure sufficient CPU and memory resources for the host and guest VMs.

4. libvirt Errors:

   • Restart the libvirtd service:

     sudo systemctl restart libvirtd
     

Conclusion

Setting up nested KVM on AlmaLinux is an invaluable skill for IT professionals, developers, and educators who rely on virtualized environments for testing and development. By following this guide, you’ve configured your system for optimal performance with nested virtualization.

From enabling hardware support to tweaking VM settings, the process ensures a robust and flexible setup tailored to your needs. AlmaLinux’s stability and compatibility with enterprise-grade features like KVM make it an excellent choice for virtualization projects.

Now, you can confidently create multi-layered virtual environments to advance your goals in testing, development, or training.