If that v86_64 thing works the way I think it does, it is not really an acceptable way to do this. If you drop the CPU down from long mode to legacy mode and get an NMI, either you are toast or you have some extremely complicated awful code to handle it. Not to mention that the CPU can’t even address all of physical memory when you do this.

Just don’t go there. Use an emulator for DOS code and use the normal kernel support (modify_ldt()) for 16-bit protected mode.