(Just to clarify an ambiguity here: by "present" I mean "exists in
memory" not "a present pte".)
So even if the guest pte were present but non-accessed, the shadow pte
would have to be non-present and you'd end up taking the fault anyway?
Hm, that does undermine the benefits. Does that mean that when the vm
clears the access bit, you always have to make the shadow non-present?
I guess so. And similarly with dirty and writable shadow.
The counter-argument is that something has gone wrong if we start
populating ptes that aren't going to be used in the near future anyway -
if they're never used then any effort taken to populate them is wasted.
Therefore, setting accessed on them from the outset isn't terribly bad.
(I'm not very convinced by that argument either, and it makes the
potential for bad side-effects much worse if the apparent RSS of a
process is multiplied by some factor.)