Re: [PATCH v2 0/2] iommu: Reserve PCI host bridge MMIO windows for IOVA

[Jason Gunthorpe <jgg@ziepe.ca>]

On Mon, Sep 21, 2026 at 12:24:13PM +0100, Robin Murphy wrote:
> On 21/09/2026 11:39 am, Guanghui Feng wrote:
> > The DMA IOVA layer reserves PCI host bridge MMIO windows via
> > iova_reserve_pci_windows() to keep IOVA allocations from overlapping
> > those address ranges.  As commit fade1ec055dc ("iommu/dma: Avoid PCI
> > host bridge windows") explains, a host bridge may interpret addresses
> > falling within its MMIO windows as peer-to-peer DMA, leading to
> > faults, data corruption, or DMA transactions being misrouted to the
> > wrong PCIe device.  This is especially dangerous when a device sits
> > behind a PCIe switch and ACS Upstream Forwarding is not fully enabled.
> > 
> > Passthrough paths that let userspace pick IOVAs did not honour these
> > windows: VFIO type1 and iommufd only excluded IOMMU driver-level
> > reserved regions (RMRR, unity maps, MSI), not PCI host bridge MMIO
> > windows.
> 
> This was intentional - I forget where the exact discussion happened, but at
> the time we decided that exposing reserved regions for entire windows was
> the wrong thing to do, as being pessimistically inaccurate might be OK for
> the internal DMA API, but restricting userspace is another matter, and I
> think the consensus was that it wasn't really the IOMMU layer's job to get
> deep into PCI details anyway. Such "accidental" P2P can only happen within a
> group that's assigned to VFIO as a whole, so IIRC the conclusion was that
> users should already have enough information at the VFIO level to avoid BAR
> addresses if they need to.

Yes, but we never did a a very good job to make the information
visible to the VFIO userspace level, and I don't think anyone ever
implemented some sysfs parsing to try to get it.

It would be a nice improvement to expose these pessimistic ranges to
userspace through iommufd so it can choose to opt into them if it
wants. Some kind of helper that the dma-iommu.c and iommufd can call
perhaps.

Jason