It doesn't need to be fully accurate, merely always conservative. It's a question of cost (how much performance does the conservative estimate waste) not feasibility.

Okay then refresh the whole thing and not just a single row whenever TRR is triggered.

Or refresh the whole thing after a certain number of row activations.

Or interleave activations with refreshes. Say, after every 5 activations, refresh the next row in the refresh cycle.

I'm not a DRAM expert. They can figure it out. I promise if you refresh the whole chip after every row activation you won't have rowhammer. It'll be too slow though. Somewhere in between is the fastest point where there isn't rowhammer.

> If it was easy they'd have done it.

Why? Something like 70->90% of the world's DRAM is produced by three companies that are very friendly to each other. It makes no business sense to fix problems that you know your peer companies will not fix... that's spending money that you absolutely do not have to.

If the business/economic theory doesn't sway you, look way back to what happened to ISP speeds and pricing when Google Fiber so much as credibly threatened to start providing service in an area served by a mono/duo/triopoly. Or -more recently- how SpaceX demonstrated that the defense-contractor-owned space launch companies had spend decades wasting enormous amounts of taxpayer money by refusing to do any significant amount of research into bringing the cost to launch down substantially. ISPs and the space launch companies had no peers that would spend the resources required to provide a better and/or cheaper service to their customers, so it made absolutely no sense for any one of them to spend resources to break the truce and make them all far less money in the long run.

It doesn't need to be a fully accurate simulation, any more than your car needs a fully accurate ICE simulation to decide "Change the engine oil".

Just count accesses to each row, and refresh the adjacent row(s) when some limit is reached.