Nested KVM: Inner VM fails to execute /init

Nested KVM: Inner VM fails to execute /init

[Steffen Gebert]

Hi all,

I started playing with nested KVMs, but the inner VM doesn't boot. It 
fails after grub's "Starting up ..." with
 > Failed to execute /init
 > Kernel panic - not syncing: No init found. Try passing init= option
 > to kernel. See Linux Documentation/init.txt for guidance.
 > Pid: 1, comm: init Not tainted 2.6.38-11-virtual #48-Ubuntu

 > Call trace is panic <- init_post <- kernel_init <-
 > kernel_thread_helper <- kernel_init <- kernel_thread_helper

Outer VM uses kernel from kvm's git repository and latest libvirt 
release (0.9.4) on Ubuntu 10.04. The inner VM uses Ubuntu's standard 
kernel + kvm.

I'm using kvm_intel (arch amd64), loaded kvm_intel with nested=1 and 
created a custom emulator script, which passes "-enable-nesting -cpu 
host" to the kvm command. I also tried "-cpu qemu64,+vmx".

Do you have a clue, why /init cannot be executed? I extracted the initrd 
image of inner VM and it looks okay. /init is executable and a readable 
shell script.

Inner and outer VMs have been built with ubuntu-vm-builder.
> ubuntu-vm-builder kvm natty --domain innervm3 --hostname innervm3 --dest innervm3 --mem 128 --addpkg acpid --addpkg openssh-server --addpkg avahi-daemon --libvirt qemu:///system
The output is exactly the same for both runs (thus no errors are printed 
out).

How can I further debug that? I must admit that I'm not that familar 
with the whole init process.

Thank you very much for your help!

Kind regards
Steffen