For what it's worth: I agree with Alex.
Aikido is constructed to work against sword, knife and stuff in between. Do your aikido correctly, and the knife is dealt with.
With reservation for the same old probability law I have mentioned before.
It is interesting to notice that anyone who does not support this above strategy (or possibility) seems to say only: if you face a knife, you're done for.
That's no doubt the very worst strategy