Qubes is upstreaming Linux stubdoms to Xen. We need to review the upstream patches and validate compatibility with OpenEmbedded and OpenXT, so we have a common upstream foundation for future improvements, e.g. PCI device isolation.
2017 blog post, MSI support for PCI device pass-through with stub domains, https://www.qubes-os.org/news/2017/10/18/msi-support/
2018/2019 in progress patches
xen: https://github.com/marmarek/xen master-linux-stubdom
Qubes issue: https://github.com/QubesOS/qubes-issues/issues/4775