PCIe Endpoint-to-Endpoint communication

I haven't done it, but i don't see why you couldn't send a memory request to another device. As long as you know the physical address of the device you want to access. I don't know if there are any software calls you will need to make to give permission for that access, but at the very least you will have to let the endpoint that wants to start the communication know the physical address of the other endpoint. It is not a limitation of the PCIe spec, and most likely not a limitation of the chipset, but it may be. Are you sure you are forming your packet correctly?