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

[Matt Evans]

Hi Kevin,

On 12/06/2026 09:46, Tian, Kevin wrote:
>> 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.

Thanks on both points!  I see what you mean, removed.


Matt