RE: [PATCH v3 2/2] vfio/type1: Set IOMMU_MMIO in dma->prot for MMIO-backed addresses

[Tian, Kevin]

> From: Jason Gunthorpe <jgg@xxxxxxxxxx>
> Sent: Friday, December 12, 2025 3:00 PM
> 
> On Fri, Dec 12, 2025 at 02:36:46AM +0000, Tian, Kevin wrote:
> > > From: Wei Wang <wei.w.wang@xxxxxxxxxxx>
> > > Sent: Thursday, November 13, 2025 11:54 PM
> > >
> > > @@ -629,6 +630,8 @@ static long vaddr_get_pfns(struct mm_struct *mm,
> > > unsigned long vaddr,
> > >  				ret = -EFAULT;
> > >  			}
> > >  		}
> > > +		if (vma->vm_flags & VM_IO)
> > > +			*prot |= IOMMU_MMIO;
> >
> > move into "if (is_invalid_reserved_pfn(*pfn)) {}". it's pointless to set
> > it in the error path.
> >
> > Reviewed-by: Kevin Tian <kevin.tian@xxxxxxxxx>
> >
> > btw another alternative is letting userspace set a map flag explicitly
> > e.g. introducing a new VFIO_DMA_MAP_FLAG_MMIO flag bit. It's
> > not considered because your use case requires existing VMMs to
> > work?
> 
> I don't think we should do that, userspace shouldn't be able to create
> a mapping inconsistent with the rest of the kernel for security
> reasons.

ah that's a good point!

> 
> I think if we ever need a cachable mmio through to the iommu then it
> should be wired through the dmabuf mechanism the same way vfio drivers
> wire the cachability when they make the mmap.
> 
> The nvgrace is the only driver that creates a cachable mmap already,
> and it would be more correct to have a cachable IOPTE, but the only HW
> tha driver works with does not care either way so it has been left.
> 
> Jason