the QEMU virtio-iommu device does support aw-bits and granule options. Those are not yet support by libvirt. Especially granule setting at libvirt level would allow to run a 4kB guest on 64kB host by forcing the granule to 4kB instead of the default matching the host's granule.

See [PATCH v6 0/3] VIRTIO-IOMMU: Set default granule to host page size and

[PATCH v3 0/3] VIRTIO-IOMMU: Introduce an aw-bits option

for more information. Note the defaults for both have changed in 9.6/10.0