Is the A bit architecturally guaranteed to be synchronously set? Can
speculative accesses set it? SDM vol 3 is a bit vague about it.
Yes. At worst Linux would underestimate the process RSS a bit
(depending on how many unsynchronized ptes you leave lying around). I
bet there's an appropriate pvop hook you could use to force
synchronization just before the kernel actually inspects the bits
(leaving lazy mode sounds good).