Usage of nested virtualization inside instances

If you want to use nested virtualization inside your compute instances you should try the parameter libvirt_cpu_mode=host-passthrough in the configuration of your compute nodes (/etc/nova/nova.conf).

While using libvirt_cpu_mode=host-model I saw the required CPU flag svm inside the instances. But loading the processor specific KVM module failed with the following messages in the kernel ring buffer.

[  664.051540] has_svm: can't execute cpuid_8000000a
[  664.051553] kvm: no hardware support

After changing the parameter libvirt_cpu_mode to host-passthrough it was possible to load the processor specific KVM module and to use KVM inside the instances.

[    1.585709] systemd[1]: Detected virtualization 'kvm'.
[    5.856891] kvm: Nested Virtualization enabled
[    5.856891] kvm: Nested Paging enabled

Of course you have to enable nested virtualization on the compute node first.

# cat /etc/modprobe.d/kvm.conf 
options kvm-intel nested=1

After reloading the CPU specific kernel module (kvm-intel or kvm-amd) with nested=1 you should get the following result.

# cat /sys/module/kvm_intel/parameters/nested

4 thoughts on “Usage of nested virtualization inside instances”

  1. I added “libvirt_cpu_mode=host-passthrough” to nova.conf, restarted all nova-* services and rebooted the VM that I want to use as an image builder, but I see nothing in the output of the guest’s dmesg that looks like what you’ve provided above. There’s nothing like it in the host’s dmesg either.

    How can I determine if nested virtualization is enabled on the VM? A “modprobe kvm” on the guest doesn’t produce errors, nor does “modprobe kvm_intel”. so the kernel modules are there. I’ve installed qemu, kvm, qemu-kvm. “kvm-ok” on the guest tells me:

    INFO: /dev/kvm exists
    KVM acceleration can be used

    When I run ‘virsh capabilities | virsh cpu-baseline /dev/stdin I get:


    VMX is there, but there’s no SVM, if it’s supposed to be there.

    Any ideas?

      1. That’s fine, but it wasn’t really my question. I’m not seeing anything in dmesg that even mentions KVM (aside from kvm-clock output). In particular, I don’t see “Nested Virtualization enabled”.

        I was just wondering if there was a way to confirm that nested virtualization was enabled on an instance in the absence of log entries.

  2. Ah, my XML pasting was eaten by a grue, apparently. The key line was:

    [feature policy='require' name='vmx']

    with the square brackets replaced by angle brackets, of course.

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