My first question is when did this start? And how many miles are on this car? I would tend to think that timing chain stretch cannot be that bad if your acceleration is still good. But if I had to guess about your mileage I would say it is probably high enough that at the very least your valve stem seals are shot and your valve guides are worn and allowing oil to enter the combustion chamber. However, this would only happen when you are not accelerating; i.e. not using the turbo boost, but when you are cruising. Now the oil seals on your turbo could be shot too. The other possibility is that since the smoke is also white, you may have a bad head gasket allowing coolant into the combustion chamber. You might check your oil for moisture; check your coolant level and check for fuel/oil in the coolant. Plus smell the exhaust. If it smells sweet your burning antifreeze. Otherwise see if you can tell if it smells like burning oil. From what I remember, Diesel burns black, oil burns gray/black and coolant is just white steam. Good luck.