If you use qemu and virtualbox on the same machine, you may encounter the following error :
VirtualBox can’t operate in VMX root mode. Please disable the KVM kernel extension, recompile your kernel and reboot (VERR_VMX_IN_VMX_ROOT_MODE).
With this message, you will be unable to run any virtual machine within Virtualbox. The root cause is either the kvm-intel or kvm-amd module has been loaded at boot time. To remove it for your current session :
root@localhost:~# lsmod |grep kvm
kvm_intel 32832 0
kvm 182683 1 kvm_intel
root@localhost:~# modprobe -r kvm_intel
No more message like the one above will be displayed.