Re: [PATCH v5 8/9] vfio/pci: Support mmap() of a VFIO DMABUF

[Pranjal Shrivastava <praan@google.com>]

On Wed, Jul 15, 2026 at 06:47:31PM +0100, Matt Evans wrote:
> A VFIO DMABUF can export a subset of a BAR to userspace by fd; add
> support for mmap() of this fd.  This provides another route for a
> process to map BARs, in which the process can only map a specific
> subset of a BAR represented by the exported DMABUF.
> 
> mmap() support enables userspace driver designs that safely delegate
> access to BAR sub-ranges to other client processes by sharing a DMABUF
> fd, without having to share the (omnipotent) VFIO device fd with them.
> 
> Since the main VFIO BAR mmap() is now DMABUF-aware, the new mmap() reuses
> the existing vm_ops.
> 
> The lifecycle of an exported DMABUF remains decoupled from that of the
> device fd it came from, i.e. the device fd could be closed with DMABUF
> VMAs present, meaning a fault on a VMA could happen concurrently with
> vfio_pci_dma_buf_cleanup().
> 
> To deal with this scenario, the fault handler now temporarily takes a
> VFIO device registration to ensure the vdev remains valid, and then
> vdev->memory_lock can be taken on it.
> 
> Signed-off-by: Matt Evans <matt@ozlabs.org>

Nice note on locking, enjoyed the lock dance!

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

Thanks,
Praan