Andrew Mercer
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Nested virtualization

Part of the KVM overview. Run a hypervisor inside a KVM guest.

Run a hypervisor inside a VM.

grep -E 'vmx|svm' /proc/cpuinfo | head -1                       # CPU flag present?
cat /sys/module/kvm_intel/parameters/nested                     # Intel: Y or 1 means enabled
cat /sys/module/kvm_amd/parameters/nested                       # AMD

Enable it if needed (with no VMs running):

echo 'options kvm_intel nested=1' | sudo tee /etc/modprobe.d/kvm-nested.conf    # kvm_amd on AMD
sudo modprobe -r kvm_intel && sudo modprobe kvm_intel

The inner hypervisor's VM must use CPU passthrough: virt-install --cpu host-passthrough ... or <cpu mode='host-passthrough'/> in the XML.