Modern GPUs allow mapping memory through PCIe, so you can treat memory as the same. Resize-able BAR effectively allows the same the other way, where all the GPU

Modern GPUs allow mapping memory through PCIe, so you can treat memory as the same. Resize-able BAR effectively allows the same the other way, where all the GPU vram can be mapped into the CPU. The issue is that suddenly accessing an address can have orders of magnitude lower bandwidth and higher latency, if it is resident on the far memory bus. So for performance reasons you still really need to treat them differently.