See AMD "Smart Memory" a.k.a. PCIe Large Bar. This expands the amount of GPU memory that the CPU can directly access, usually to the GPU's entire memory range (ordinarily only ~256Mb is accessible). GPU->CPU Reads have very high latencies, but that's not an issue for CPU->GPU writes.

GPUs have been able to access "host" memory for a long time now, with a few restrictions: you have to setup the GPU mappings first and pin the pages in memory.