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

[Leon Romanovsky <leon@kernel.org>]

On Tue, May 05, 2026 at 08:50:58AM -0600, Alex Williamson wrote:
> 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,

Right, there was no need to remove ZONE_DEVICE when I added my code, and I
left the task of cleaning it out of is_pci_p2pdma_page() for another day.
Without ZONE_DEVICE, all pages are treated as non‑P2P.

> 
> Alex
> 
> PS - Please also weigh in on the dmabuf underflow[1]
> 
> [1]https://lore.kernel.org/all/20260501131236.278ac431@shazbot.org/

will do.

Thanks