> hence the suggestions that BPF programs could be submitted through the ring to chain operations all within the kernel
That sounds unbelievably annoying.
No, the limit has been found and it is not in the kernel. You push the I/O loop up for userspace I/O, more specific, not more general. They call it SPDK [1] (or DPDK for networking) and as far I can tell, the principle is essentially having a dummy driver in the kernel that maps the entire PCIe peripheral memory space into your chosen process, and everything flows from there.
At the I/O limit, asynchronous isn't feasible because interrupts introduce latency and waste cycles not doing work. All userspace I/O frameworks work only through polling.