mistranslation: 0.939 instruction: 0.939 other: 0.853 device: 0.830 semantic: 0.808 graphic: 0.753 assembly: 0.691 network: 0.669 socket: 0.634 vnc: 0.580 boot: 0.460 KVM: 0.448 lock mov non generated #UD qemu 2.8.1 debian 9.1 Could you please add a proper description to this bug? Please also try first whether your problem also occurs with the latest released version of QEMU (version 2.9 or the 2.10 release candidate), to see whether it has been fixed there already. sorry i english poor： intel manual say： The LOCK prefix can be prepended only to the following instructions and only to those forms of the instructions where the destination operand is a memory operand: ADD, ADC, AND, BTC, BTR, BTS, CMPXCHG, CMPXCH8B, CMPXCHG16B, DEC, INC, NEG, NOT, OR, SBB, SUB, XOR, XADD, and XCHG. If the LOCK prefix is used with one of these instructions and the source operand is a memory operand, an undefined opcode exception (#UD) may be generated. An undefined opcode exception will also be generated if the LOCK prefix is used with any instruction not in the above list. The XCHG instruction always asserts the LOCK# signal regardless of the presence or absence of the LOCK prefix. but qemu NO! lock mov forms of the instructions non generated #UD exception! my OS : debian 9.1 QEMU: qemu 2.8.1 At 2017-08-21 12:54:44, "Thomas Huth"