Re: [PATCH v4 10/10] vfio/pci: Add dma-buf export support for MMIO regions

[Alex Williamson <alex.williamson@redhat.com>]

On Tue, 30 Sep 2025 11:34:08 -0300
Jason Gunthorpe <jgg@nvidia.com> wrote:

> On Tue, Sep 30, 2025 at 12:50:47PM +0000, Shameer Kolothum wrote:
> 
> > This is where hisi_acc reports a different BAR size as it tries to hide
> > the migration control region from Guest access.  
> 
> I think for now we should disable DMABUF for any PCI driver that
> implements a VFIO_DEVICE_GET_REGION_INFO
> 
> For a while I've wanted to further reduce the use of the ioctl
> multiplexer, so maybe this series:
> 
> https://github.com/jgunthorpe/linux/commits/vfio_get_region_info_op/
> 
> And then the dmabuf code can check if the ops are set to the generic
> or not and disable itself automatically.
> 
> Otherwise perhaps route the dmabuf through an op and deliberately omit
> it (with a comment!) from hisi, virtio, nvgrace.
> 
> We need to route it through an op anyhow as those three drivers will
> probably eventually want to implement their own version.

Can't we basically achieve the same by testing the ioctl is
vfio_pci_core_ioctl?  Your proposal would have better granularity, but
we'd probably want an ops callback that we can use without a userspace
buffer to get the advertised region size if we ever want to support a
device that both modifies the size of the region relative to the BAR
and supports p2p.  Thanks,

Alex