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

[Wei Wang]

On 12/12/25 3:26 PM, Tian, Kevin wrote:
> > 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.

Yeah, that looks better, thanks.

> > > 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