Problem enabling IOMMU with Buster and Xen

[Intense Red <intnsred@golgotha.net>]

   A box booting Xen with an updated Debian 10/Buster setup as Xen's dom0, a 
Ryzen CPU and Radeon-based GPU.

   /etc/default/grub has been modified to include:

GRUB_CMDLINE_LINUX_DEFAULT="quiet iommu=1 amd_iommu=on"

and grub has been updated.

   After a reboot Xen reports:

# xl dmesg | grep IOMMU
(XEN) AMD-Vi: IOMMU Extended Features:
(XEN) AMD-Vi: IOMMU 0 Enabled.

   Cool. But GNU/Linux says:

# dmesg | grep IOMMU
[    2.536053] AMD IOMMUv2 driver by Joerg Roedel <jroedel@suse.de>
[    2.536054] AMD IOMMUv2 functionality not available on this system

   WTF?! Can someone whack me with a clue-bat? TIA.

-- 
"It is a damn poor mind indeed which can't think of at least two ways to spell 
any word." -- US President Andrew Jackson.