Comment # 5 on bug 109135 from iive@yahoo.com

(In reply to rmuncrief from comment #2)
[...]
> Here's the grub line I use for all testing:
> GRUB_CMDLINE_LINUX_DEFAULT="quiet
> resume=UUID=b4f71480-8fe3-43c2-99c9-fc3f5687545b libata.atapi_passthru16=0
> rd.modules-load=vfio-pci amd_iommu=on iommu=pt amdgpu.modeset=1
[...]

Would you try with "iommu=soft", aka disable the hardware one?
Maybe try tuning it off entirely.