Thank you for the clarification Bjorn! I was on vacation..sorry for my delay.
Closing the loop here, I understand we're not getting this patch merged (due to its restriction to domain 0) and there was a suggestion in the thread of trying to block MSIs from the IOMMU init code (which also have the restriction of only working in iommu-enabled systems). Hope the thread is helpful and if somebody faces this issue, can comment here and at least find this approach, maybe test the patch. Thanks to all involved!
