Re: [PATCH v13 5/5] RDMA/mlx5: get tph for p2p access when registering dma-buf mr

[Jason Gunthorpe <jgg@ziepe.ca>]

On Thu, Sep 24, 2026 at 11:17:57PM -0700, Zhiping Zhang wrote:
> On Thu, Sep 24, 2026 at 4:31 PM Jason Gunthorpe <jgg@ziepe.ca> wrote:
> >
> > >
> > On Wed, Sep 23, 2026 at 11:24:56PM -0700, Zhiping Zhang wrote:
> > > I don't think this belongs in the importer. Every in-tree dma-buf
> > > caller of pci_p2pdma_distance() is the exporter, in its .attach,
> > > clearing attach->peer2peer -- amdgpu_dma_buf.c, xe_dma_buf.c,
> > > habanalabs memory.c. There is no importer-side caller.
> >
> > Right, and they shouldn't be doing that, but it still has to be
> > checked that the st is going directly to the peer device not the host
> > bridge. Not using the distance, but by the PCI_P2PDMA_MAP_BUS_ADDR
> > indication.
> >
> > I fear you will need some of Leon's series to make that happen.
> >
> > So probably the proposed change to dmabuf ops is far too simple.
> >
> > Jason
> 
> Hi Jason,
> 
> Thanks for the comments.
> 
> Agreed -- the tag should not be handed out unless the routing is
> PCI_P2PDMA_MAP_BUS_ADDR. That can be enforced conservatively on 7.3
> with two changes. I can fold both into patch 4.
> ```
> In drivers/pci/p2pdma.c, make pci_p2pdma_map_type() callable from
> tristate VFIO_PCI_CORE:
> 
>     EXPORT_SYMBOL_GPL(pci_p2pdma_map_type);

No, that's been rejected several times already.
 
> and in drivers/vfio/pci/vfio_pci_dmabuf.c,
> vfio_pci_dma_buf_get_pci_tph() gains:
> 
>     struct dma_buf_attachment *attach;
> 
>     if (list_empty(&dmabuf->attachments))
>         return -EOPNOTSUPP;
> 
>     list_for_each_entry(attach, &dmabuf->attachments, node)
>         if (pci_p2pdma_map_type(priv->provider, attach->dev) !=
>             PCI_P2PDMA_MAP_BUS_ADDR)
>                 return -EOPNOTSUPP;
> ```

Also no, this has to be done via PCI_P2PDMA_MAP_BUS_ADDR which also
enforces putting the determination in the right place in the code
flow..

> Two properties are worth stating explicitly:

Ah! AI!

Jason