Re: [PATCH 1/2] vfio/pci: Set up VFIO barmap before creating a DMABUF

[Alex Williamson <alex@shazbot.org>]

On Wed, 15 Apr 2026 11:14:22 -0700
Matt Evans <mattev@meta.com> wrote:

> A DMABUF exports access to BAR resources which need to be requested
> before the DMABUF is handed out.  Usually the resources are requested
> when setting up the barmap when the VFIO device fd is mmap()ed, but
> there's no guarantee that's done before a DMABUF is created.
> 
> Set up the barmap (and so request resources) in the DMABUF-creation
> path.
> 
> Fixes: 5d74781ebc86c ("vfio/pci: Add dma-buf export support for MMIO regions")
> Signed-off-by: Matt Evans <mattev@meta.com>
> ---
>  drivers/vfio/pci/vfio_pci_dmabuf.c | 9 +++++++++
>  1 file changed, 9 insertions(+)
> 
> diff --git a/drivers/vfio/pci/vfio_pci_dmabuf.c b/drivers/vfio/pci/vfio_pci_dmabuf.c
> index 4ccaf3531e02..fefe7cf4256b 100644
> --- a/drivers/vfio/pci/vfio_pci_dmabuf.c
> +++ b/drivers/vfio/pci/vfio_pci_dmabuf.c
> @@ -272,6 +272,15 @@ int vfio_pci_core_feature_dma_buf(struct vfio_pci_core_device *vdev, u32 flags,
>  		goto err_free_priv;
>  	}
>  
> +	/*
> +	 * See comment in vfio_pci_core_mmap(); ensure PCI regions
> +	 * were requested before returning DMABUFs that reference
> +	 * them.  Barmap setup does this:
> +	 */
> +	ret = vfio_pci_core_setup_barmap(vdev, get_dma_buf.region_index);
> +	if (ret)
> +		goto err_free_phys;
> +
>  	priv->vdev = vdev;
>  	priv->nr_ranges = get_dma_buf.nr_ranges;
>  	priv->size = length;

Wouldn't this get a lot easier if we just setup all the barmaps in
vfio_pci_core_enable(), conditional on pci_resource_len() just like we
use to filter in REGION_INFO?

I don't recall if there's some reason we've avoid this so far, maybe
others can shout it out if they do.

We already tear them all down in vfio_pci_core_disable().  It would be
a small patch to add that, which we would mark as Fixes:, then a small
follow-up on top of that that removes any then redundant or unnecessary
callers (all of them).  Thoughts?  Thanks,

Alex

BTW, cover letters are appreciated for multi-part series.