RE: [PATCH v3 4/9] vfio/pci: Convert BAR mmap() to use a DMABUF

[Tian, Kevin]

> From: Matt Evans <matt@xxxxxxxxxx>
> Sent: Wednesday, June 10, 2026 11:43 PM
> 
> Convert the VFIO device fd fops->mmap to create a DMABUF representing
> the BAR mapping, and make the VMA fault handler look up PFNs from the
> corresponding DMABUF.  This supports future code mmap()ing BAR
> DMABUFs, and iommufd work to support Type1 P2P.
> 
> First, vfio_pci_core_mmap() uses the new
> vfio_pci_core_mmap_prep_dmabuf() helper to export a DMABUF
> representing a single BAR range.  Then, the vfio_pci_mmap_huge_fault()
> callback is updated to understand revoked buffers, and uses the new
> vfio_pci_dma_buf_find_pfn() helper to determine the PFN for a given
> fault address.
> 
> Now that the VFIO DMABUFs can be mmap()ed, vfio_pci_dma_buf_move()
> zaps PTEs (used on the revocation and cleanup paths).
> 
> CONFIG_VFIO_PCI_CORE now unconditionally depends on
> CONFIG_DMA_SHARED_BUFFER and CONFIG_PCI_P2PDMA_CORE.  The
> CONFIG_VFIO_PCI_DMABUF feature conditionally includes support for
> VFIO_DEVICE_FEATURE_DMA_BUF, depending on the availability of
> CONFIG_PCI_P2PDMA.
> 
> Signed-off-by: Matt Evans <matt@xxxxxxxxxx>

Reviewed-by: Kevin Tian <kevin.tian@xxxxxxxxx>

with a nit:

> -	vma->vm_private_data = vdev;
> +	/*
> +	 * Create a DMABUF with a single range corresponding to this
> +	 * mapping, and wire it into vma->vm_private_data.  The VMA's
> +	 * vm_file becomes that of the DMABUF, and the DMABUF takes
> +	 * ownership of the VFIO device file (put upon DMABUF
> +	 * release).  This maintains the behaviour of a live VMA
> +	 * mapping holding the VFIO device file open.
> +	 */
> +	ret = vfio_pci_core_mmap_prep_dmabuf(vdev, vma,
> +					     pci_resource_start(pdev, index),
> +					     req_len, index);

the comment is redundant as it's about internal logic of the callee
and is well covered by the comment there.