Re: [PATCH v7 00/11] vfio/pci: Allow MMIO regions to be exported through dma-buf

[Jason Gunthorpe <jgg@ziepe.ca>]

On Tue, Nov 18, 2025 at 03:37:41PM +0100, Christian König wrote:

> Skimming over it my only concern is patch #6 which adds the helper
> to the common DMA-buf code and that in turn would need an in-deep
> review which I currently don't have time for.

I think you should trust Leon on the implementation. He knows what he
is doing here when it comes to the DMA API, since he made all the
patches so far to use it.

Please consider just reviewing the exported function signature:

+struct sg_table *dma_buf_map(struct dma_buf_attachment *attach,
+			     struct p2pdma_provider *provider,
+			     struct dma_buf_phys_vec *phys_vec,
+			     size_t nr_ranges, size_t size,
+			     enum dma_data_direction dir)

If issues are discovered inside the implementation later on then Leon
will be available to fix them.

The code is intended to implement that basic function signature which
can be thought of as dma_map_resource() done correctly for PCI
devices.

> So if we could keep those inside the VFIO driver for now I think
> that should be good to go.

That was several versions ago. Christoph is very strongly against
this, he wants to see the new DMA API used by wrapper functions in
subsytems related to how the subsystem's data structures work rather
than proliferate into drivers. I agree with this, so we need to go in
this direction.

Other options, like put the code in the DMA API area, are also not
going to be agreed because we really don't want this weird DMABUF use
of no-struct page scatterlist to leak out beyond DMABUF.

So, this is the start of a DMA mapping helper API for DMABUF related
data structures, it introduces a simplified mapping entry point for
drivers that only use MMIO.

As I said I expect this API surface to progress as other DRM drivers
are updated (hopefully DRM community will take on this), but there is
nothing wrong with starting by having a basic entry point for a narrow
use case.

Thanks,
Jason