Re: [PATCH v4 1/5] PCI/P2PDMA: Don't enforce ACS check for device functions of Intel GPUs

["Thomas Hellström" <thomas.hellstrom@linux.intel.com>]

On Thu, 2025-09-25 at 10:33 -0300, Jason Gunthorpe wrote:
> On Thu, Sep 25, 2025 at 03:11:50PM +0200, Thomas Hellström wrote:
> > On Thu, 2025-09-25 at 13:28 +0200, Christian König wrote:
> > > On 25.09.25 12:51, Thomas Hellström wrote:
> > > > > > In that case I strongly suggest to add a private DMA-buf
> > > > > > interface
> > > > > > for the DMA-
> > > > > > bufs exported by vfio-pci which returns which BAR and
> > > > > > offset
> > > > > > the
> > > > > > DMA-buf
> > > > > > represents.
> > > > 
> > > > @Christian, Is what you're referring to here the "dma_buf
> > > > private
> > > > interconnect" we've been discussing previously, now only
> > > > between
> > > > vfio-
> > > > pci and any interested importers instead of private to a known
> > > > exporter
> > > > and importer?
> > > > 
> > > > If so I have a POC I can post as an RFC on a way to negotiate
> > > > such
> > > > an
> > > > interconnect.
> > > 
> > > I was just about to write something up as well, but feel free to
> > > go
> > > ahead if you already have something.
> > 
> > Just posted a POC. It might be that you have better ideas, though.
> 
> I think is also needs an API that is not based on scatterlist. Please
> lets not push a private interconnect address through the scatterlist
> dma_addr_t!

I think that needs to be defined per interconnect, choosing a data
structure that suits best. Although I find it reasonable to mandate
dma_addr_t or scatterlists to *not* be used.

This merely focuses on the interconnect negotiation itself.

> 
> Assuming that you imagine we'd define some global well known
> interconnect
> 
> 'struct blah pci_bar_interconnect {..}'
> 
> And if that is negotiated then the non-scatterlist communication
> would
> give the (struct pci_dev *, bar index, bar offset) list?

Yes something like that. Although I think perhaps the dev + bar index
might be part of the negotiation, so that it is rejected if the
importer feels that there is no implied PF + VF interconnect. Then the
list would be reduced to only the offset.

Still I think Vivek would be better to figure the exact negotiation and
data structure out.
 
/Thomas

> 
> I think this could solve the kvm/iommufd problems at least!
> 
> Jason