That's not true, the kernel driver knows what page mappings belong to particular processes, including GPU page mappings. Moreover, you have no choice but to tal

That's not true, the kernel driver knows what page mappings belong to particular processes, including GPU page mappings. Moreover, you have no choice but to talk to that kernel driver; you can't go behind its back and talk truly directly to the GPU, even if you bypass the userspace GPU driver, because this would allow circumventing memory protection. It is true, however, that modern GPU kernel drivers are relatively thin.