Enable KVM for virt-v2v (Nested Virtualization)

Overview

vJailbreak runs virt-v2v inside a Kubernetes pod (v2v-helper) to convert VM disks and inject drivers. By default, virt-v2v uses QEMU as its conversion backend. QEMU can run in two modes:

Mode	How it works	Performance
KVM (hardware-accelerated)	Uses /dev/kvm — the host kernel’s KVM module	Fast
TCG (software emulation)	Pure software fallback, no /dev/kvm required	~10–20× slower

When /dev/kvm is not visible inside the pod, QEMU falls back to TCG and logs:

qemu-kvm: Could not access KVM kernel module: No such file or directory
qemu-kvm: falling back to tcg

Conversions still succeed but take significantly longer.

How vJailbreak exposes /dev/kvm

The v2v-helper pod already mounts the host /dev directory into the pod at /dev via a hostPath volume. This means if /dev/kvm exists on the vJailbreak VM, it is automatically visible inside the pod — no additional configuration is required.

The condition that must be met is that the vJailbreak VM itself has access to KVM, which requires nested virtualization to be enabled at every layer of the stack.

Enabling Nested Virtualization

Layer 1 — Hypervisor (host running the vJailbreak VM)

The physical host (or outer hypervisor) must expose hardware virtualization extensions to its guest VMs.

KVM/QEMU host

Use host-passthrough or host-model CPU mode, or explicitly add vmx (Intel) / svm (AMD) to the CPU flags:

<cpu mode="host-passthrough"/>

OpenStack

Set the flavor’s hw:cpu_mode to host-passthrough or enable the hw:nested_virt trait:

openstack flavor set <flavor> --property hw:cpu_mode=host-passthrough

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Layer 2 — vJailbreak VM (the Linux OS)

Once the hypervisor exposes the virtualization extensions, verify that the KVM kernel module is loaded inside the vJailbreak VM:

# For Intel CPUs
lsmod | grep kvm_intel

# For AMD CPUs
lsmod | grep kvm_amd

If the module is not loaded, load it manually (and add to /etc/modules to persist across reboots):

# Intel
modprobe kvm_intel
echo kvm_intel >> /etc/modules

# AMD
modprobe kvm_amd
echo kvm_amd >> /etc/modules

Verify /dev/kvm is present:

ls -l /dev/kvm
# Expected: crw-rw---- 1 root kvm ...

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Layer 3 — Pod (automatic)

No changes are needed. The pod’s /dev hostPath mount makes /dev/kvm visible to the v2v-helper container as soon as it exists on the vJailbreak VM.

Verifying KVM is in use

After enabling nested virtualization and restarting a migration, inspect the v2v-helper pod logs. You should not see the TCG fallback warning. Instead, look for QEMU initializing with KVM:

kubectl -n migration-system logs <migration-name>-v2v-helper | grep -i kvm

A healthy KVM-accelerated run shows no falling back to tcg messages and noticeably faster conversion times.

Troubleshooting

Symptom	Likely cause	Fix
Could not access KVM kernel module in logs	/dev/kvm missing on vJailbreak VM	Enable nested virt on the outer hypervisor and load the KVM module
kvm_intel/kvm_amd module fails to load	Virtualization extensions not exposed by hypervisor	Configure the outer hypervisor to pass through CPU virt flags
/dev/kvm present on VM but not in pod	Unlikely — the hostPath mount covers all of /dev	Confirm the pod spec has the /dev hostPath volume (default in vJailbreak)