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

[Pavel Begunkov <asml.silence@gmail.com>]

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.

It took a couple of iterations on the list to get it to the current
design, v2 of the series can be looked up at [1], which implements the
infrastructure and initial wiring for NVMe. It slightly diverges from
the description above, as some of the framework bits are block specific,
and I'll be working on refining that and simplifying some of the
interfaces for v3. A good chunk of block handling is based on prior work
from Keith that was pre DMA mapping buffers [2].

Tushar was helping and mention he got good numbers for P2P transfers
compared to bouncing it via RAM. Anuj, Kanchan and Nitesh also
previously reported encouraging results for system memory backed
dma-buf for optimising IOMMU overhead, quoting Anuj:

- STRICT: before = 570 KIOPS, after = 5.01 MIOPS
- LAZY: before = 1.93 MIOPS, after = 5.01 MIOPS
- PASSTHROUGH: before = 5.01 MIOPS, after = 5.01 MIOPS

[1] https://lore.kernel.org/io-uring/cover.1763725387.git.asml.silence@gmail.com/
[2] https://lore.kernel.org/io-uring/20220805162444.3985535-1-kbusch@fb.com/
-- 
Pavel Begunkov