Re: [PATCH v3 00/10] Add dmabuf read/write via io_uring

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

Hey Ming,

On 5/4/26 16:29, Ming Lei wrote:
> On Wed, Apr 29, 2026 at 04:25:46PM +0100, Pavel Begunkov wrote:
>> The patch set allows to register a dmabuf to an io_uring instance for
>> a specified file and use it with io_uring read / write requests. The
>> infrastructure is not tied to io_uring and there could be more users
>> in the future. A similar idea was attempted some years ago by Keith [1],
>> from where I borrowed a good number of changes, and later was brough up
>> by Tushar and Vishal from Intel.
>>
>> It's an opt-in feature for files, and they need to implement a new
>> file operation to use it. Only NVMe block devices are supported in this
>> series. The user API is built on top of io_uring's "registered buffers",
>> where a dmabuf is registered in a special way, but after it can be used
>> as any other "registered buffer" with IORING_OP_{READ,WRITE}_FIXED
>> requests. It's created via a new file operation and the resulted map is
>> then passed through the I/O stack in a new iterator type. There is some
>> additional infrastructure to bind it all, which also counts requests
>> using a dmabuf map and managing lifetimes, which is used to implement
>> map invalidation.
>>
>> It was tested for GPU <-> NVMe transfers. Also, as it maintains a
>> long-term dma mapping, it helps with the IOMMU cost. The numbers
>> below are for udmabuf reads previously run by Anuj for different
>> IOMMU modes:
> 
> Plain registered buffer is long-live too, which raises question: does this
> framework need to take it into account from beginning?

Not sure I follow, mind expanding on what should be accounted?
Are you suggesting that we might want to use normal registered
buffers in a similar way? I.e. giving the driver an ability to
pre-register them?

> BTW, inspired by this approach, I adds similar feature to ublk via UBLK_IO_F_SHMEM_ZC
> which can maintain long-term vfio dma mapping over registered user-place aligned buffer.

Interesting, just too a glance, and it looks like what David Wei
was thinking to add to fuse, but IIUC he gave up exactly because the
client will need to cooperate and that could be troublesome.

Should we try to push everything under the same interface instead of
keeping a ublk specific one? Again to the point that it requires
a cooperative client, but if it's something more generic, the user
might just try to use it as a general optimisation. In the same way
it'll be helpful to fuse, and as a bonus you wouldn't need tree look
ups (but mandates clients using registered buffers as a downside).

It'd need to shaped to somehow work better with host memory as I
assume you want to be able to map it into server in common case.
Switch case'ing if it's a udmabuf is not the greatest approach,
but maybe we can figure out something else.
  
-- 
Pavel Begunkov