RDMA-capable NICs enable kernel bypass by setting up direct communication channels between user-space applications and the hardware with shared memory region in the application memory space.
Setup (Kernel): The kernel maps shared memory regions (Send Queues/SQs, Completion Queues/CQs) and "doorbell" registers directly into user space.
Send (User-Space): Application writes Work Requests (WRs) to the SQ, then signals the NIC via a memory-mapped doorbell write (no syscall). The NIC uses DMA to fetch WRs and data directly from host memory.
Completion (User-Space Fast Path): NIC writes Completion Queue Entries (CQEs) to the CQ. The application polls the CQ directly for status (no syscall again...).
This allows zero-copy, low-latency data transfer by using DMA and direct hardware signaling, bypassing the kernel for data path operations.