code: func (d *Device) entHdrOp(off int) tcmuOpcode {
  	i := int(*(*uint32)(unsafe.Pointer(&d.mmap[off+offLenOp])))
  	i = i & 0x7
  	return tcmuOpcode(i)
  }
docs: /*
  
  // Only a few opcodes, and length is 8-byte aligned, so use low bits for opcode.
  struct tcmu_cmd_entry_hdr {
    __u32 len_op;
    __u16 cmd_id;
    __u8 kflags;
  #define TCMU_UFLAG_UNKNOWN_OP 0x1
    __u8 uflags;
  
  } __packed;
  */