Re: [RFC PATCH 3/7] vfio/pci: Support mmap() of a DMABUF

[Jason Gunthorpe]

On Fri, Feb 27, 2026 at 01:52:15PM -0800, Alex Mastro wrote:
> On Fri, Feb 27, 2026 at 03:48:07PM -0400, Jason Gunthorpe wrote:
> > > > I actually would like to go the other way and have VFIO always have a
> > > > DMABUF under the VMA's it mmaps because that will make it easy to
> > > > finish the type1 emulation which requires finding dmabufs for the
> > > > VMAs.
> > 
> > This is a still better idea since it avoid duplicating the VMA flow
> > into two parts..
> 
> I suppose this would also compose with your idea to use dma-buf for
> iommufd_compat support of VFIO_IOMMU_MAP_DMA of vfio device fd-backed mmap()s
> [1]? Instead of needing to materialize a new dma-buf, you could use the existing
> backing one?

Yeah, that too

I think it is a fairly easy progression:

1) mmap_prepare() allocates a new dmabuf file * and sticks it in
   desc->vm_file. Rework so all the vma_ops are using vm_file that is
   a dmabuf. The allocated dmabuf has a singleton range
2) Teach the fault handlers to support full range semantics
3) Use dmabuf revoke variables/etc in the mmap fault handlers
4) Move the address space from the vfio to the dmabuf
5) Allow mmaping the dmabuf fd directly which is now only a couple lines

I forget how all the different mmap implementations in vfio interact
though - but I think the above is good for vfio-pci

Jason