"When a game is running, the CPU needs access to a graphic card’s memory, and by default it can only access 256MB at a time. However, resizable BAR removes the size limitation so the CPU has access to all of a GPU’s memory, theoretically allowing for improved performance. This adjustment lets the CPU request much larger chunks of data from the GPU, which can improve performance in games that can benefit from this change."
Yes, I'm aware of it. It's purely an addressing simplification and you are still limited by the GPU bus speed if you want to access any data on the GPU by the CPU. And it only works one way, the GPU cannot be given an address in RAM and expected to magically have access to it, the driver has to copy the data over first.
"When a game is running, the CPU needs access to a graphic card’s memory, and by default it can only access 256MB at a time. However, resizable BAR removes the size limitation so the CPU has access to all of a GPU’s memory, theoretically allowing for improved performance. This adjustment lets the CPU request much larger chunks of data from the GPU, which can improve performance in games that can benefit from this change."