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.

https://images.anandtech.com/doci/7677/03%20-%20HSA%20Featur...

https://images.anandtech.com/doci/7677/17%20-%20hUMA%20Benef...

https://www.anandtech.com/show/7677/amd-kaveri-review-a8-760...