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

[Matt Evans]

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@xxxxxxxxxx>
>> ---
>>  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).

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

>>  	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).)

> Apart from these, 
> 
> Reviewed-by: Pranjal Shrivastava <praan@xxxxxxxxxx>


Thanks!


Matt