>    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.

Nikhil