Re: [PATCH 1/9] vfio/pci: Add vfio_pci_dma_buf_iommufd_map()

[Nicolin Chen <nicolinc@nvidia.com>]

On Fri, Nov 07, 2025 at 12:49:33PM -0400, Jason Gunthorpe wrote:
> This function is used to establish the "private interconnect" between the
> VFIO DMABUF exporter and the iommufd DMABUF importer. This is intended to
> be a temporary API until the core DMABUF interface is improved to natively
> support a private interconnect and revocable negotiation.
> 
> This function should only be called by iommufd when trying to map a
> DMABUF. For now iommufd will only support VFIO DMABUFs.
> 
> The following improvements are needed in the DMABUF API to generically
> support more exporters with iommufd/kvm type importers that cannot use the
> DMA API:
> 
>  1) Revoke semantics. VFIO needs to be able to prevent access to the MMIO
>     during FLR, and so it will use dma_buf_move_notify() to prevent
>     access. iommmufd does not support fault handling so it cannot
>     implement the full move_notify. Instead if revoke is negotiated the
>     exporter promises not to use move_notify() unless the importer can
>     experiance failures. iommufd will unmap the dmabuf from the iommu page
>     tables while it is revoked.
> 
>  2) Private interconnect negotiation. iommufd will only be able to map
>     a "private interconnect" that provides a phys_addr_t and a
>     struct p2pdma_provider * to describe the memory. It cannot use a DMA
>     mapped scatterlist since it is directly calling iommu_map().
> 
>  3) NULL device during dma_buf_dynamic_attach(). Since iommufd doesn't use
>     the DMA API it doesn't have a DMAable struct device to pass here.
> 
> Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
 
Reviewed-by: Nicolin Chen <nicolinc@nvidia.com>