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Summary

Conventional NIC transmit paths send a small MMIO doorbell to the device and then rely on DMA reads to fetch descriptors and packet buffers. This paper revisits that design point and argues that direct MMIO writes can be a shorter, faster CPU-to-NIC path when ordering constraints are relaxed.

The work proposes hardware support at the NIC to recover the ordering needed for correctness while allowing CPUs to issue high-throughput unordered MMIO writes. This makes a simple programmed-I/O transmit path competitive with more complex DMA-centric interfaces.

Bibtex

@inproceedings{liew:mmio,
  author = {Wei Siew Liew and Md Ashfaqur Rahaman and James McMahon and Ryan Stutsman and Vijay Nagarajan},
  title = {{Stop Taking the Scenic Route: the Shortest Distance Between the CPU and the NIC is MMIO}},
  booktitle = {Proceedings of the 2025 Workshop on Hot Topics in Operating Systems},
  series = {HotOS '25},
  year = {2025},
  pages = {144--150},
  publisher = {ACM},
  url = {https://doi.org/10.1145/3713082.3730389},
  doi = {10.1145/3713082.3730389},
}

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