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To run a virtual machine inside a KVM guest, enable or verify nested virtualization on the physical KVM host (L0), expose the required CPU virtualization features to the guest hypervisor (L1), and confirm that KVM—not QEMU’s TCG software emulation—is accelerating L1. The virtual machine started by L1 is L2. Nested virtualization is documented as enabled by default on Intel and AMD with Linux kernel 4.20 and later, but a distribution can override that setting.
Identify the virtualization layers first
In this setup, L0 is the physical Linux host running KVM, L1 is the virtual machine running a hypervisor, and L2 is a VM created by L1. Nested virtualization lets a guest run another guest; the inner hypervisor can be KVM or a different hypervisor, as the Linux kernel’s nested-guest guide explains.
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Confirm that these are the layers you mean before troubleshooting. A VM running under QEMU with TCG emulation is not the same as KVM accelerated inside a guest, even if the VM starts successfully.
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The Linux kernel documents nested virtualization as enabled by default for Intel and AMD beginning with kernel v4.20. That is a default, not a guarantee for every distribution or running host: module configuration can override it. Check the active setting on L0 rather than relying on the documented default.
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- For Intel: run
cat /sys/module/kvm_intel/parameters/nestedon L0. - For AMD: run
cat /sys/module/kvm_amd/parameters/nestedon L0. - Check the output against your distribution’s kernel and module configuration. If nesting is disabled, consult that distribution’s instructions for making the setting persistent and applying it.
Changing a module parameter may require reloading the KVM module. Do not unload an in-use host module casually: first determine whether VMs are running and follow the distribution’s operational guidance. The kernel guide covers the parameter and configuration details in its nested KVM documentation.
Expose virtualization features to L1
Enabling nesting on L0 is not enough by itself. L1 must receive the CPU virtualization features its hypervisor needs. Configure the CPU model presented by the QEMU or libvirt setup that launches L1, then verify what L1 actually sees.
| CPU exposure choice | When it fits | Trade-off |
|---|---|---|
QEMU -cpu host |
When L1 should see the host CPU’s available capabilities. | Exposes host capabilities; this may be unsuitable as a stable CPU baseline for migration across different hosts. |
| A named CPU model with the required virtualization feature enabled | When a more consistent CPU baseline is important, such as for migration planning. | The model and feature set must be supported by the host, QEMU/libvirt configuration, and L1’s requirements. |
There is no universally correct CPU model: the right choice depends on host capabilities, software versions, and migration requirements. Confirm feature exposure from inside L1 instead of assuming that a setting on L0 was passed through. The kernel guide gives CPU configuration examples and caveats in Running nested guests with KVM.
Verify that L1 is using KVM acceleration
If L1 cannot launch L2, first establish whether KVM is active inside L1. QEMU can run with TCG emulation instead; that is not KVM-on-KVM acceleration. Check that /dev/kvm exists in L1, that the KVM modules are loaded there, and that the VM launch configuration actually enables KVM acceleration. A VM booting is not proof that it is using KVM.
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Also check that L1 sees the virtualization extensions and CPU features needed by its hypervisor. If those features are absent, revisit both the L0 nested setting and the CPU model used to launch L1.
Troubleshoot by where the failure occurs
- L1 cannot see virtualization features: recheck the active nesting parameter on L0 and the CPU model used for L1. Verify the feature visibility inside L1.
- L1 sees the features, but its VMs are not accelerated: check for
/dev/kvmand confirm that L1’s QEMU or hypervisor is configured to use KVM rather than TCG. - L2 will not boot: verify that L1’s hypervisor supports the required feature set and that the L0 and L1 configuration is consistent. Not every VMX feature is fully supported by KVM.
- L2 boots but performs poorly: check the vendor-specific performance settings below and compare the actual hardware and software versions at both levels. The available kernel documentation does not establish a universal nested-virtualization overhead figure.
- Migration or save/restore fails: check the CPU vendor, whether L2 is running, and the Linux kernel and QEMU versions against the documented limitations below.
Investigate slow nested guests without assuming a speedup
For Intel systems, the kernel’s guidance points to settings including EPT and Shadow VMCS when investigating a slow L2. It also describes Shadow VMCS and APIC virtualization in connection with sufficiently capable hardware. Treat these as diagnostic checks, not a guaranteed performance fix: results depend on the platform and configuration.
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The cited documentation provides no workload-independent performance penalty or expected percentage. Compare the exact L0 and L1 hardware, kernel, QEMU, and hypervisor configuration before attributing slowness to nesting itself. See the kernel’s Nested VMX documentation for implementation context.
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Check vendor and version limits before migration
Migration behavior depends on vendor, software versions, and whether a nested guest is active. The kernel documentation distinguishes these cases:
| Situation documented by the Linux kernel | Guidance |
|---|---|
| Intel x86 L1 has an active L2 | Migration support is documented as working with Linux kernel 5.3 and QEMU 4.2.0 and later. |
| AMD L1 has started an L2 | Do not migrate L1 or save and restore it until L2 has shut down; the documented result is undefined and may be unstable. |
The kernel guide also states that L2 migration is expected to work in the scenarios it specifies. These are version-sensitive implementation notes, not a substitute for validating the exact production stack. Review the current kernel migration guidance before designing a migration procedure.
Collect useful evidence when the problem persists
A useful report should let someone trace both the outer and inner hypervisor configurations. Collect the following from L0 and L1 where applicable:
- Linux kernel, QEMU, and libvirt versions.
- The complete QEMU command lines used to launch L1 and L2.
- CPU information and
lscpuoutput at both levels. - Full
dmesgoutput from L0 and L1. - On x86, the kernel guide additionally suggests
x86info -aanddmidecodeoutput from both levels.
Include the CPU vendor and state whether L2 was running when the failure occurred. That distinction matters especially for migration and save/restore issues.
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KVM aims to provide a standard VMX implementation, but the kernel’s Nested VMX documentation notes that not all VMX features are fully supported. The CPU virtualization limitations page also documents an AMD nested SVM debug-exception behavior that KVM does not fully virtualize. A nested hypervisor therefore should not be assumed to behave identically to one running directly on bare metal.
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