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

[Ming Lei]

On Wed, May 06, 2026 at 10:02:11AM +0100, Pavel Begunkov wrote:
> 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?

Yeah, normal registered buffer is long-live too, which is exactly
what the driver cares for the long-term dma mapping motivation.

> 
> > 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.

Here the cooperation is minimized, maybe one shmem/hugetlb path, or memfd,
and it is one optimization and opt-in, and fallback to normal path
if application doesn't cooperate.

> 
> Should we try to push everything under the same interface instead of
> keeping a ublk specific one? Again to the point that it requires

If generic interface can be figured out, it shouldn't be a big deal for
ublk to switch to it, and the usage is simple actually.

So far, ublk supports both FS and nvme block device.

And cooperation can't be avoided for this usage no matter if generic or
driver specific implementation is taken, for both fuse & ublk.

> 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).

Yeah, but tree lookup is fast enough in case of huge page for typical
application, and it is simple in concept.


Thanks,
Ming