> That sounds a bit like a performance counter to me, but it does > raise an interesting question whether "idempotent" in the > architectural sense is idempotent in a mathematical sense > (i.e. operations are repeatable with the same result) or in a > broader sense (e.g. inclusive of any side-effects even if the > values at the location don't change).
Whether one treats a performance counter as a 'benign side effect' or not depends on what one does with it. It could be merely passive instrumentation to obtaim meta-information about program behavior. But if the counter itself is observable to the program and can affect its future behavior, then it's more serious.
I would avoid _requiring_ anything about itempotency in the spec, and just leave it to commentary like this for the system designer to be aware of the issues and to decide whether PTWs in their system can happen in idempotent regions or not.
