Re: [PATCH v2 1/9] vfio/pci: Add vfio_pci_dma_buf_iommufd_map()

[Alex Williamson <alex@shazbot.org>]

On Fri, 21 Nov 2025 11:50:58 -0400
Jason Gunthorpe <jgg@nvidia.com> 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.
> 
> Reviewed-by: Nicolin Chen <nicolinc@nvidia.com>
> Reviewed-by: Kevin Tian <kevin.tian@intel.com>
> Tested-by: Nicolin Chen <nicolinc@nvidia.com>
> Tested-by: Shuai Xue <xueshuai@linux.alibaba.com>
> Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
> ---
>  drivers/vfio/pci/vfio_pci_dmabuf.c | 34 ++++++++++++++++++++++++++++++
>  include/linux/vfio_pci_core.h      |  4 ++++
>  2 files changed, 38 insertions(+)
> 
> diff --git a/drivers/vfio/pci/vfio_pci_dmabuf.c b/drivers/vfio/pci/vfio_pci_dmabuf.c
> index 6698f540bdac87..d4d0f7d08c53e2 100644
> --- a/drivers/vfio/pci/vfio_pci_dmabuf.c
> +++ b/drivers/vfio/pci/vfio_pci_dmabuf.c
> @@ -82,6 +82,40 @@ static const struct dma_buf_ops vfio_pci_dmabuf_ops = {
>  	.release = vfio_pci_dma_buf_release,
>  };
>  
> +/*
> + * This is a temporary "private interconnect" between VFIO DMABUF and iommufd.
> + * It allows the two co-operating drivers to exchange the physical address of
> + * the BAR. This is to be replaced with a formal DMABUF system for negotiated
> + * interconnect types.
> + *
> + * If this function succeeds the following are true:
> + *  - There is one physical range and it is pointing to MMIO
> + *  - When move_notify is called it means revoke, not move, vfio_dma_buf_map
> + *    will fail if it is currently revoked
> + */
> +int vfio_pci_dma_buf_iommufd_map(struct dma_buf_attachment *attachment,
> +				 struct dma_buf_phys_vec *phys)
> +{
> +	struct vfio_pci_dma_buf *priv;
> +
> +	dma_resv_assert_held(attachment->dmabuf->resv);
> +
> +	if (attachment->dmabuf->ops != &vfio_pci_dmabuf_ops)
> +		return -EOPNOTSUPP;
> +
> +	priv = attachment->dmabuf->priv;
> +	if (priv->revoked)
> +		return -ENODEV;
> +
> +	/* More than one range to iommufd will require proper DMABUF support */
> +	if (priv->nr_ranges != 1)
> +		return -EOPNOTSUPP;
> +
> +	*phys = priv->phys_vec[0];
> +	return 0;
> +}
> +EXPORT_SYMBOL_FOR_MODULES(vfio_pci_dma_buf_iommufd_map, "iommufd");
> +
>  int vfio_pci_core_fill_phys_vec(struct dma_buf_phys_vec *phys_vec,
>  				struct vfio_region_dma_range *dma_ranges,
>  				size_t nr_ranges, phys_addr_t start,
> diff --git a/include/linux/vfio_pci_core.h b/include/linux/vfio_pci_core.h
> index c9466ba323fa9c..6a3074f2cf1cea 100644
> --- a/include/linux/vfio_pci_core.h
> +++ b/include/linux/vfio_pci_core.h
> @@ -28,6 +28,7 @@ struct vfio_pci_core_device;
>  struct vfio_pci_region;
>  struct p2pdma_provider;
>  struct dma_buf_phys_vec;
> +struct dma_buf_attachment;
>  
>  struct vfio_pci_regops {
>  	ssize_t (*rw)(struct vfio_pci_core_device *vdev, char __user *buf,
> @@ -203,4 +204,7 @@ VFIO_IOREAD_DECLARATION(32)
>  VFIO_IOREAD_DECLARATION(64)
>  #endif
>  
> +int vfio_pci_dma_buf_iommufd_map(struct dma_buf_attachment *attachment,
> +				 struct dma_buf_phys_vec *phys);
> +
>  #endif /* VFIO_PCI_CORE_H */

Acked-by: Alex Williamson <alex@shazbot.org>