Zero-copy network transmission with io_uring

What is the reason behind the hoop jumping with extra notifications and generations and userspace informing the kernel when it can move on to next generations?

With the completion notifications, it would have seemed like the simplest, most resistant to misuse API would be to delay the completion notification until it was *actually* done - which is what we wanted? (Unless there is some semantic meaning worth differentiating between the two that I'm missing.)