AMD's APUs or the Windows Display Driver Model did not support USM until very recently. They would explicitly carve out memory at boot for graphics use and physical pages could not be shared between GPU and CPU. They'd still make use of apertures to copy data in/out of respective memory ranges.
That is simply not true. Kavari (2014) had pointer passing between CPU & GPU. I don't know where you're getting that pages couldn't be shared from but it's not accurate.
