> That's fine, but what percent of players of X game are going to have such a setup in their computer?

If the "put some CPU cores on the GPU" thing becomes popular, probably a lot.

> I'd like a source for that if you have one. I'd be very surprised if modern IOMMU implementations with paging need arbitrary access. The CPU / OS could presumably modify the IOMMU entries prior to the DMA swap. The OS is still the one initiating a DMA transaction.

Traditional paging implementations didn't use IOMMU at all -- a lot of machines don't even physically have one, and even the ones that have one, that doesn't mean the OS is using it for that. It might end up going through it if you have something like the storage controller is mapped as a device to a VM guest and then the host uses the IOMMU to map the storage controller's DMA to the memory pages corresponding to what the guest perceives as its physical memory, or things along those lines.

But remapping the pages for each access, even if theoretically possible, would be pretty expensive. Page table operations aren't cheap and have significant synchronization overhead, and to swap a page that way would require you to both map the page and then almost immediately do another operation to unmap it again. For each 4kB page, since they're unlikely to be contiguous. You can do the math on how many page table operations that would add if you were swapping in, say, 500MiB, which a modern SSD could otherwise do in tens of milliseconds. Notice in particular that this would make operating systems that do this get lower scores in benchmarks. And that this applies not just to swap as a result of being out of memory, but ordinary file accesses which are really a swap to the page cache.

You could also run into trouble if you tried to do that because the IOMMU may only support a finite number of mappings, or have performance issues if you create too many. Then you get a slow device with too many pending I/O operations and the whole system locks up.

And even if you paid the cost, what have you bought? The OS could still give a device access to any given memory page for legitimate reasons and you have no way to know if the reason was the legitimate one or the user arranged for those circumstances to exist so they could access the page.