Talking with GPUs works a lot like this using modern APIs. You fill a command buffer with stuff and then asynchronously get results back "later", using synchron

Talking with GPUs works a lot like this using modern APIs. You fill a command buffer with stuff and then asynchronously get results back "later", using synchronization primitives and memory mapping. There are syscalls to kick off the command buffers but those can (and often are) be implemented in userspace by the driver.