The SMM on some laptops thunks to real mode to run certain INT10 functions like adjust brightness and switch displays. It is not clear how SMM can do this when virtualization is enabled. Does SMM stop and restart the virtual machine monitor when it thunks to real mode?
Here is what I came up with from the engineers I spoke with:
Unless you have a parallel monitor set up (STM), you no longer operate as either VT-x guest or root. When you return from SMM, you go back to either root or guest. (In some ways, SMM is sort of a precursor to VT-x root mode.) You can thunk to real mode as long as you are in SMM.
More info on SMM interactions with various modes can be found in chapter 25 (System Management) of volume 3B of the Intel Instruction Set Reference for Core 2 Duo.
http://download.intel.com/design/processor/manuals/253669.pdf