Reading more the VM* command sets definitely load/save more than just the normally visible registers, the descriptions in the AMD ASM manual are very explicit about that. However, it looks like (outside the encrypted guest case where everything is done in 1 command) the hyper visor still calls the typical XRSTOR for the float registers, which is no different than the normal OS case. If that's true then I can see how the register file is still contaminated in the non SMT case.