> approach of using the reserved space only for readonly pages until needed,
> and then kicking the pages out doesn't require any flushing and could
> probably be done without switching thread context.
> 
> Either mechanism on its own could probably be called a solution; if both
> were available you could accept reservations for snappy attaches but also
> be forgiving if the reservation was underestimated.

And maybe there's a kind of heuristic hybrid: identify the physical address
ranges that are most favorable for bus DMA mapping purposes given the buses
and architecture configured, and preferentially try to map readonly pages
in those ranges, whether there has been an explicit reservation or not.

-Chap