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

[Pranjal Shrivastava <praan@google.com>]

On Wed, Jun 10, 2026 at 04:43:18PM +0100, Matt Evans wrote:
> 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@ozlabs.org>
> ---
>  drivers/vfio/pci/Kconfig           |  5 +-
>  drivers/vfio/pci/Makefile          |  3 +-
>  drivers/vfio/pci/vfio_pci_core.c   | 75 +++++++++++++++++++-----------
>  drivers/vfio/pci/vfio_pci_dmabuf.c | 12 +++++
>  drivers/vfio/pci/vfio_pci_priv.h   | 11 +----
>  5 files changed, 67 insertions(+), 39 deletions(-)
> 
> diff --git a/drivers/vfio/pci/Kconfig b/drivers/vfio/pci/Kconfig
> index 296bf01e185e..67a2ae1fbc04 100644
> --- a/drivers/vfio/pci/Kconfig
> +++ b/drivers/vfio/pci/Kconfig
> @@ -6,6 +6,8 @@ config VFIO_PCI_CORE
>  	tristate
>  	select VFIO_VIRQFD
>  	select IRQ_BYPASS_MANAGER
> +	select PCI_P2PDMA_CORE
> +	select DMA_SHARED_BUFFER
>  
>  config VFIO_PCI_INTX
>  	def_bool y if !S390
> @@ -56,7 +58,8 @@ config VFIO_PCI_ZDEV_KVM
>  	  To enable s390x KVM vfio-pci extensions, say Y.
>  
>  config VFIO_PCI_DMABUF
> -	def_bool y if VFIO_PCI_CORE && PCI_P2PDMA && DMA_SHARED_BUFFER
> +	def_bool y if PCI_P2PDMA
> +	depends on VFIO_PCI_CORE
>  
>  source "drivers/vfio/pci/mlx5/Kconfig"
>  
[...]  
>  int vfio_pci_core_mmap_prep_dmabuf(struct vfio_pci_core_device *vdev,
>  				   struct vm_area_struct *vma,
> @@ -532,6 +538,10 @@ void vfio_pci_dma_buf_move(struct vfio_pci_core_device *vdev, bool revoked)
>  	struct vfio_pci_dma_buf *tmp;
>  
>  	lockdep_assert_held_write(&vdev->memory_lock);
> +	/*
> +	 * Holding memory_lock ensures a racing VMA fault observes
> +	 * priv->revoked properly.
> +	 */

Nit: This comment should appear before the lockdep_assert_held_write()
Also, it is slightly verbose.. (not against it though).

>  
>  	list_for_each_entry_safe(priv, tmp, &vdev->dmabufs, dmabufs_elm) {
>  		if (!get_file_active(&priv->dmabuf->file))
> @@ -549,6 +559,8 @@ void vfio_pci_dma_buf_move(struct vfio_pci_core_device *vdev, bool revoked)
>  			if (revoked) {
>  				kref_put(&priv->kref, vfio_pci_dma_buf_done);
>  				wait_for_completion(&priv->comp);
> +				unmap_mapping_range(priv->dmabuf->file->f_mapping,
> +						    0, priv->size, 1);

Have we run this series with lockdep enabled?
I guess it'd be nice to check with lockdep once..

Apart from these, 

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

Thanks,
Praan