Re: [LSF/MM/BPF TOPIC] dmabuf backed read/write

[Nitesh Shetty <nj.shetty@samsung.com>]

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On 03/02/26 02:29PM, Pavel Begunkov wrote:
>Good day everyone,
>
>dma-buf is a powerful abstraction for managing buffers and DMA mappings,
>and there is growing interest in extending it to the read/write path to
>enable device-to-device transfers without bouncing data through system
>memory. I was encouraged to submit it to LSF/MM/BPF as that might be
>useful to mull over details and what capabilities and features people
>may need.
>
>The proposal consists of two parts. The first is a small in-kernel
>framework that allows a dma-buf to be registered against a given file
>and returns an object representing a DMA mapping. The actual mapping
>creation is delegated to the target subsystem (e.g. NVMe). This
>abstraction centralises request accounting, mapping management, dynamic
>recreation, etc. The resulting mapping object is passed through the I/O
>stack via a new iov_iter type.
>
>As for the user API, a dma-buf is installed as an io_uring registered
>buffer for a specific file. Once registered, the buffer can be used by
>read / write io_uring requests as normal. io_uring will enforce that the
>buffer is only used with "compatible files", which is for now restricted
>to the target registration file, but will be expanded in the future.
>Notably, io_uring is a consumer of the framework rather than a
>dependency, and the infrastructure can be reused.
>
We have been following the series, its interesting from couple of angles,
- IOPS wise we see a major improvement especially for IOMMU
- Series provides a way to do p2pdma to accelerator memory

Here are few topics which I am looking into specifically,
- Right now the series uses a PRP list. We need a good way to keep the
   sg_table info around and decide on‑the‑fly whether to expose the buffer
   as a PRP list or an SG list, depending on the I/O size.
- Possibility of futher optimization for new type of iov iter to reduce
   per IO cost

Thanks,
Nitesh

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