linux-kernel - Re: [RFC v2 00/11] Add dmabuf read/write via io

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Message-ID: <fd10fe48-f278-4ed0-b96b-c4f5a91b7f95@amd.com>
Date: Mon, 24 Nov 2025 11:33:03 +0100
From: Christian König <christian.koenig@....com>
To: Pavel Begunkov <asml.silence@...il.com>, linux-block@...r.kernel.org,
 io-uring@...r.kernel.org
Cc: Vishal Verma <vishal1.verma@...el.com>, tushar.gohad@...el.com,
 Keith Busch <kbusch@...nel.org>, Jens Axboe <axboe@...nel.dk>,
 Christoph Hellwig <hch@....de>, Sagi Grimberg <sagi@...mberg.me>,
 Alexander Viro <viro@...iv.linux.org.uk>,
 Christian Brauner <brauner@...nel.org>,
 Andrew Morton <akpm@...ux-foundation.org>,
 Sumit Semwal <sumit.semwal@...aro.org>, linux-kernel@...r.kernel.org,
 linux-nvme@...ts.infradead.org, linux-fsdevel@...r.kernel.org,
 linux-media@...r.kernel.org, dri-devel@...ts.freedesktop.org,
 linaro-mm-sig@...ts.linaro.org
Subject: Re: [RFC v2 00/11] Add dmabuf read/write via io_uring

On 11/23/25 23:51, Pavel Begunkov wrote:
> Picking up the work on supporting dmabuf in the read/write path.

IIRC that work was completely stopped because it violated core dma_fence and DMA-buf rules and after some private discussion was considered not doable in general.

Or am I mixing something up here? Since I don't see any dma_fence implementation at all that might actually be the case.

On the other hand we have direct I/O from DMA-buf working for quite a while, just not upstream and without io_uring support.

Regards,
Christian.

> There
> are two main changes. First, it doesn't pass a dma addresss directly by
> rather wraps it into an opaque structure, which is extended and
> understood by the target driver.
> 
> The second big change is support for dynamic attachments, which added a
> good part of complexity (see Patch 5). I kept the main machinery in nvme
> at first, but move_notify can ask to kill the dma mapping asynchronously,
> and any new IO would need to wait during submission, thus it was moved
> to blk-mq. That also introduced an extra callback layer b/w driver and
> blk-mq.
> 
> There are some rough corners, and I'm not perfectly happy about the
> complexity and layering. For v3 I'll try to move the waiting up in the
> stack to io_uring wrapped into library helpers.
> 
> For now, I'm interested what is the best way to test move_notify? And
> how dma_resv_reserve_fences() errors should be handled in move_notify?
> 
> The uapi didn't change, after registration it looks like a normal
> io_uring registered buffer and can be used as such. Only non-vectored
> fixed reads/writes are allowed. Pseudo code:
> 
> // registration
> reg_buf_idx = 0;
> io_uring_update_buffer(ring, reg_buf_idx, { dma_buf_fd, file_fd });
> 
> // request creation
> io_uring_prep_read_fixed(sqe, file_fd, buffer_offset,
>                          buffer_size, file_offset, reg_buf_idx);
> 
> And as previously, a good bunch of code was taken from Keith's series [1].
> 
> liburing based example:
> 
> git: https://github.com/isilence/liburing.git dmabuf-rw
> link: https://github.com/isilence/liburing/tree/dmabuf-rw
> 
> [1] https://lore.kernel.org/io-uring/20220805162444.3985535-1-kbusch@fb.com/
> 
> Pavel Begunkov (11):
>   file: add callback for pre-mapping dmabuf
>   iov_iter: introduce iter type for pre-registered dma
>   block: move around bio flagging helpers
>   block: introduce dma token backed bio type
>   block: add infra to handle dmabuf tokens
>   nvme-pci: add support for dmabuf reggistration
>   nvme-pci: implement dma_token backed requests
>   io_uring/rsrc: add imu flags
>   io_uring/rsrc: extended reg buffer registration
>   io_uring/rsrc: add dmabuf-backed buffer registeration
>   io_uring/rsrc: implement dmabuf regbuf import
> 
>  block/Makefile                   |   1 +
>  block/bdev.c                     |  14 ++
>  block/bio.c                      |  21 +++
>  block/blk-merge.c                |  23 +++
>  block/blk-mq-dma-token.c         | 236 +++++++++++++++++++++++++++++++
>  block/blk-mq.c                   |  20 +++
>  block/blk.h                      |   3 +-
>  block/fops.c                     |   3 +
>  drivers/nvme/host/pci.c          | 217 ++++++++++++++++++++++++++++
>  include/linux/bio.h              |  49 ++++---
>  include/linux/blk-mq-dma-token.h |  60 ++++++++
>  include/linux/blk-mq.h           |  21 +++
>  include/linux/blk_types.h        |   8 +-
>  include/linux/blkdev.h           |   3 +
>  include/linux/dma_token.h        |  35 +++++
>  include/linux/fs.h               |   4 +
>  include/linux/uio.h              |  10 ++
>  include/uapi/linux/io_uring.h    |  13 +-
>  io_uring/rsrc.c                  | 201 +++++++++++++++++++++++---
>  io_uring/rsrc.h                  |  23 ++-
>  io_uring/rw.c                    |   7 +-
>  lib/iov_iter.c                   |  30 +++-
>  22 files changed, 948 insertions(+), 54 deletions(-)
>  create mode 100644 block/blk-mq-dma-token.c
>  create mode 100644 include/linux/blk-mq-dma-token.h
>  create mode 100644 include/linux/dma_token.h
>