Thinking a bit about that… it would seem to be the hypervisor’s job to handle IOMMU, and in fact, /var/log/xen/console/hypervisor.log does show:
[2021-09-11 17:04:58] (XEN) AMD-Vi: IOMMU 0 Enabled.
[2021-09-11 17:04:58] (XEN) I/O virtualisation enabled
[2021-09-11 17:04:58] (XEN) - Dom0 mode: Relaxed
So of the iommu is already managed by the hypervisor what’s taking place in dom0 that needs to have iommu drivers there too, @marmarek ?