Nested Virtual Machines with KVM, libvirt, and virsh

AdvancedLinux~4 hours
KVMlibvirtvirt-installvirsh

I enabled nested virtualization on KVM and provisioned guest VMs inside a VM with libvirt, virt-install, and virsh. Nesting let me build multi-node clustering and hypervisor labs on a single physical host with no additional hardware.

Objective & Context

Nested KVM exposes the CPU virtualization extensions to a guest so it can itself run VMs. This lab turns on nesting, scripts guest creation with virt-install, and manages lifecycle with virsh, the technique behind the Proxmox and Kubernetes cluster labs.

Environment & Prerequisites

  • Linux host with KVM, libvirt, and CPU virtualization extensions.
  • Sufficient RAM/CPU for nested guests.
  • A cloud or ISO image for the inner guest.

Step-by-Step Execution

1. Enable nested virtualization [ROOT REQUIRED]

modprobe -r kvm_intel; modprobe kvm_intel nested=1; cat /sys/module/kvm_intel/parameters/nested

2. Provision an inner guest with virt-install

virt-install --name inner1 --memory 2048 --vcpus 2 --disk size=10 --osinfo debian12 --import

3. Manage with virsh

virsh list --all && virsh start inner1
Y   # nested enabled
 Id   Name     State
 3    inner1   running

Validation & Testing

Confirm nested reads Y, then boot an inner guest and verify it sees virtualization extensions (egrep -c '(vmx|svm)' /proc/cpuinfo > 0). Pass criteria: nesting enabled, inner guest boots, and it can itself run a container or VM.

Advanced: Troubleshooting
  • nested = N: the module must be reloaded with nested=1; persist via modprobe.d.
  • Inner guest no VT: set the L1 VM CPU mode to host-passthrough.
  • Poor performance: nesting adds overhead; allocate generous host resources.

Key Results

  • Enabled nested KVM and booted guests inside a VM.
  • Built multi-node lab topologies on a single physical host.
  • Scripted reproducible guest creation with virt-install.
  • Managed full VM lifecycle through virsh.

Related Labs