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

[Pranjal Shrivastava <praan@google.com>]

On Fri, Jun 12, 2026 at 04:22:12PM +0100, Matt Evans wrote:
> Hi Pranjal,
> 
> On 12/06/2026 11:41, Pranjal Shrivastava wrote:
> > 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(-)

Hi Matt,

[...]  
> >>  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).
> 
> Right, I'll move it.  Agree it's wordy but if anyone changes that I want
> them to "think faulthandler".
> 

That's fair I guess.

> >>  	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..
> 
> I've (generally) always run testing of this series with lockdep.  (No
> issues (anymore).)

That sounds good! Thanks for confirming! :)

Praan