@tomg That is the work, but it needs rebasing over the XSA-400 work, so a v4 series is going to be needed at a minimum.
HAP is Xen's vendor-neutral name for Intel EPT or AMD NPT hardware support. We have had superpage support for many years here.
IOMMU pagetables can either be shared with EPT/NPT (reduces the memory overhead of running the VM), or split (required for AMD due to hardware incompatibilities, and also required to support migration of a VM with an IO devices).
When pagetables are shared, the HAP superpage support gives the IOMMU superpages too (because they're literally the same set of pagetables in memory). When pagetables are split, HAP gets superpages while the IOMMU logic currently uses small pages.