Monthly Archive for October, 2013

Nested Virt and Fedora 20 Virt Test Day


I decided this year to take part in the Fedora Virtualization Test Day on October 8th. In order to take part I needed a system with Fedora 20 installed so that I could then create VMs on top. Since I like my current setup and I didn't have a hard drive laying around that I wanted to wipe I decided to give nested virtualization a shot.

Most of the documentation I have seen for nested virtualization has come from Kashyap Chamarthy. Relevant posts are here, here, and here. He has done a great job with these tutorials and this post is nothing more than my notes for what I found to work for me.


With nested virtualization the OS/Hypervisor that touches the physical hardware is known as L0. The first level of virtualized guest is known as L1. The second level of virtualized guest (the guest inside a guest) is known as L2. In my setup I ultimately wanted F19(L0), F20(L1), and F20(L2).

First, in order to pass along intel vmx extensions to the guest I created a modprobe config file that instructs the kvm_intel kernel module to allow nested virtualization support:

[root@L0 ~]# echo "options kvm-intel nested=y" > /etc/modprobe.d/nestvirt.conf

After a reboot I can now confirm the kvm_intel moduel is configured for nested virt:

[root@L0 ~]# cat /sys/module/kvm_intel/parameters/nested Y

Next I converted an existing Fedora 20 installation to use "host-passthrough" (see here) so that the L1 guest would see the same processor (with vmx extensions) as my L0 host. To do this i modified the cpu xml tags as follows in the libvirt xml definition:

<cpu mode='host-passthrough'> </cpu>

After powering up the guest I now see that the processor that the L1 guest sees is indeed the same as the host:
[root@L1 ~]# cat /proc/cpuinfo | grep "model name" model name : Intel(R) Core(TM) i7-3770 CPU @ 3.40GHz model name : Intel(R) Core(TM) i7-3770 CPU @ 3.40GHz model name : Intel(R) Core(TM) i7-3770 CPU @ 3.40GHz model name : Intel(R) Core(TM) i7-3770 CPU @ 3.40GHz

Next I decided to enable nested virt in the L1 guest by adding the same modprobe.conf file as I did in L0. I did this based on a tip from Kashyap in the #fedora-test-day chat that this tends to give about a 10X performance improvement in the L2 guests.

[root@L1 ~]# echo "options kvm-intel nested=y" > /etc/modprobe.d/nestvirt.conf

After a reboot I could then create and install L2 guests using virt-install and virt-manager. This seemed to work fine except for the fact that I would often see an unknown NMI in the guest periodically.

[ 14.324786] Uhhuh. NMI received for unknown reason 30 on CPU 0. [ 14.325046] Do you have a strange power saving mode enabled? [ 14.325046] Dazed and confused, but trying to continue

I believe the issue I was seeing may be documented in kernel BZ#58941 . After asking about it in the chat I was informed that for the best experience with nested virt I should go to a 3.12 kernel. I decided to leave that exercise for another day :).

Have a great day!