Re: virtio-iommu issue with VFIO device downstream to a PCIe-to-PCI bridge: VFIO devices are not assigned any iommu group

[Jean-Philippe Brucker <jean-philippe@linaro.org>]

On Wed, Jan 18, 2023 at 11:28:32AM -0700, Alex Williamson wrote:
> The VT-d spec[2](8.3.1) has a more elegant solution using a path
> described in a device scope, based on a root bus number (not
> susceptible to OS renumbering) and a sequence of devfns to uniquely
> describe a hierarchy or endpoint, invariant of OS bus renumbering.

That's a good idea, we could describe the hierarchy using only devfns.
I think I based VIOT mostly on IORT and device-tree which don't provide
that as far as I know, but could have studied DMAR better. One problem is
that for virtio-iommu we'd need to update both device-tree and VIOT (and
neither are easy to change).

But it's worth thinking about because it would solve a problem we
currently have, that a virtio-iommu using the virtio-pci transport cannot
be placed behind a bridge, including a root port, because the firmware
tables cannot refer to it.

Thanks,
Jean