Virtualbox erroneously requires VMX

[Bernhard M.]

When using VirtualBox on a VMX enabled Core2 CPU, that is already running KVM, there is always an error on VM start - even when the checkbox for "VT-x/AMD-V" is disabled:

Failed to start the virtual machine winxp. 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).

Result Code: NS_ERROR_FAILURE (0x80004005) Component: Console Interface: IConsole {9511bc54-15ee-4ddf-808e-472aba03809c}

The same thing worked on AMD/SVM (since 2.1.2). Software Virtualization should just work even when VMX is unavailable.

Host is Debian/lenny x86_64

[Sander van Leeuwen]

Wrong conclusion. You can't use either VT-x or software virtualization if KVM is active. This check prevents you from killing your machine when you try anyway.

[Sander van Leeuwen]

It works with AMD-V, because such restrictions don't apply there.

[Bernhard M.]

I just fail to see how KVM can possibly interfere with software virtualization. Through CPU registers/flags that are mandatory for software-virtualization?

[Sander van Leeuwen]

You are not allowed to turn off paging in VMX root mode. Clearly described in the Intel documentation and a well known issue with VT-x.

This limitation prevents you from executing 32 bits guests on a 64 bits host or 32 bits PAE guests on a 32 bits non-PAE host. In order to make our life simpler we simply don't allow it at all.