But in the paper they explicitly test the possibility that the undocumented instructions could be speculatively executed and have visible side effects. They fo

But in the paper they explicitly test the possibility that the undocumented instructions could be speculatively executed and have visible side effects. They found that the instructions are actually executed, but they couldn’t jump to arbitrary microcode (a side effect of one of the undocumented instructions) because of a single lfence ucode op. They proved that at least on the atom processor they could unlock, the speculative execution of the undocumented instruction calculated an offset and placed it in internal registers even though the instruction was executed from the “green” (normal) operating mode.