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

[Alex Williamson <alex@shazbot.org>]

On Tue, 5 May 2026 13:49:11 +0300
Leon Romanovsky <leon@kernel.org> wrote:

> On Mon, May 04, 2026 at 04:40:41AM -0300, Jason Gunthorpe wrote:
> > On Fri, May 01, 2026 at 04:19:15PM -0600, Alex Williamson wrote:
> >   
> > > Exporting dma-bufs from vfio-pci is a feature, but mmap of MMIO BARs is
> > > a legacy requirement.  That legacy requirement now depends on
> > > PCI_P2PDMA, which depends on 64BIT and ZONE_DEVICE.  
> > 
> > That should be split up now, Leon missed it when he added the new
> > APIs that didn't require ZONE_DEVICE..  
> 
> Sorry, what did I miss here?  
> VFIO_DMABUF is an optional feature and is enabled only when P2P support is  
> available. It does not affect legacy systems where P2P cannot be enabled.

If we look at the long term view of moving exclusively to cdev/iommufd,
where VFIO_DMABUF becomes the mechanism for implementing P2P DMA
mappings, VFIO_DMABUF may be optional, but it's highly desirable for
legacy compatibility.  There's an argument though that providing P2P
compatibility on platforms that support PCI_P2PDMA is probably
sufficient.

However, in providing mmap of dmabufs as a feature, this series is
wiring all mmaps through dmabufs and therefore that dependency becomes
fundamental to the use of vfio-pci.  Thus the discussion whether the
noted config requirements could be lifted.  Thanks,

Alex

PS - Please also weigh in on the dmabuf underflow[1]

[1]https://lore.kernel.org/all/20260501131236.278ac431@shazbot.org/