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

Jason Gunthorpe
2026-02-09T13:21:47Z

On Mon, Feb 09, 2026 at 10:59:53AM +0000, Pavel Begunkov wrote:

> > As a step forward I could imagine having a DMABUF handing out P2P
> > pages and allowing io uring to "register" it complete with move
>
> Forcing dma-buf to have pages is a big step back, IMHO

Naw, some drivers already have them anyhow, and we are already looking
at optional ways to allow a very limited select group of importers to
access the underlying physical.

It is not a big leap from there to say io_uring pre-registration is a
special importer that only interworks with drivers providing P2P
pages.

It could immediately address everything except pre-registration. And
do you really care about pre-registration? Why? Running performance
workloads with the iommu doing a DMA mapping is pretty unusual.

> > Pre-iommu-mapping the pool seems like an orthogonal project as it
> > applies to everything coming from pre-registered io uring buffers,
> > even normal cpu memory. You could have a next step of pre-mapping the
> > P2P pages and CPU pages equally.
>
> It was already tried for normal user memory (not by me), but
> the verdict was that it should be dma-buf based.

I'm not sure how DMA-buf helps anything here. It is the io uring layer
that should be interacting with DMA-buf, the lower level stuff
shouldn't touch it.

Jason