Re: [PATCH v3 3/9] vfio/pci: Add a helper to create a DMABUF for a BAR-map VMA

[Pranjal Shrivastava <praan@google.com>]

On Wed, Jun 10, 2026 at 04:43:17PM +0100, Matt Evans wrote:
> This helper, vfio_pci_core_mmap_prep_dmabuf(), creates a single-range
> DMABUF for the purpose of mapping a PCI BAR.  This is used in a future
> commit by VFIO's ordinary mmap() path.
> 
> This function transfers ownership of the VFIO device fd to the
> DMABUF, which fput()s when it's released.
> 
> Refactor the existing vfio_pci_core_feature_dma_buf() to split out
> export code common to the two paths, VFIO_DEVICE_FEATURE_DMA_BUF and
> this new VFIO_BAR mmap().
> 
> Signed-off-by: Matt Evans <matt@ozlabs.org>
> ---
>  drivers/vfio/pci/vfio_pci_dmabuf.c | 142 +++++++++++++++++++++++------
>  drivers/vfio/pci/vfio_pci_priv.h   |   5 +
>  2 files changed, 117 insertions(+), 30 deletions(-)
>

[...]

> +
> +	/*
> +	 * Ownership of the DMABUF file transfers to the VMA so that
> +	 * other users can locate the DMABUF via a VA.  Ownership of
> +	 * the original VFIO device file being mmap()ed transfers to
> +	 * priv, and is put when the DMABUF is released.  This
> +	 * intentionally does not use get_file()/vma_set_file()
> +	 * because the references are already held, and ownership
> +	 * moves.
> +	 */
> +	priv->vfile = vma->vm_file;
> +	vma->vm_file = priv->dmabuf->file;
> +	vma->vm_private_data = priv;

I appreciate this comment. Thanks for being clear!

Reviewed-by: Pranjal Shrivastava <praan@google.com>

Thanks,
Praan