Cannot turn iommu on inside guest

So I want to turn iommu support on in a guest ( <type arch='x86_64' machine='pc-q35-4.1'>hvm</type>). Per [1] I tried to add to devices either <devices> <iommu model='intel'> <driver intremap='on'/> </iommu> </devices> or just a simpler <devices> <iommu model='intel /'> </devices> However, when I try to boot it, it crashes with the following error message (stolen from /var/log/libvirt/qemu/testkvm.log): 2020-02-11T05:13:27.278707Z qemu-system-x86_64: We need to set caching-mode=on for intel-iommu to enable device assignment with IOMMU protection. 2020-02-11 05:13:27.577+0000: shutting down, reason=crashed that sounds like it wants me to do something like qemu-system-x86_64 [...] -device intel-iommu,intremap=on,caching-mode=on[2] using virsh. But <devices> <iommu model='intel'> <driver intremap='on' caching-mode='on '/> </iommu> </devices> is not only not mentioned in [1] but also fails [root@testbox tests]# virsh edit testkvm error: XML document failed to validate against schema: Unable to validate doc against /usr/share/libvirt/schemas/domain.rng Extra element devices in interleave Element domain failed to validate content Failed. Try again? [y,n,i,f,?]: Can anyone show me what I am missing here? [1] https://libvirt.org/formatdomain.html#elementsIommu [2] https://wiki.qemu.org/Features/VT-d#Command_Line_Example_2