Comment 1 for bug 1915081

This issue no longer exists in Ubuntu. Kernel bisection shows that it impacted upstream kernels between v4.20 and v5.3.

Bisection was a little complicated because there are 2 overlapping issues. There's the reboot hang, but there's also an issue that causes the host mellanox driver to crash when you passthrough a VF. So I bisected the mellanox driver crash first, then manually applied that fix while biscting the reboot hang.

Here's a chronological set of the relevant commits (annotation will
require a fixed-width font):

v4.19
975bb8b4dc93 PCI/IOV: Use VF0 cached config space size for other VFs ------------------+
v4.20-rc1 |
b61d271e59d7 iommu/dma: Move domain lookup into __iommu_dma_{map,unmap} --------+ +---- Reboot hangs
76bf6a8634a1 Revert "PCI/IOV: Use VF0 cached config space size for other VFs" --|------+
v5.3-rc1 +--- Passthrough crashes
8af23fad6261 iommu/dma: Handle MSI mappings separately -------------------------+
v5.3-rc5

As you can see, the reboot hang problem started in upstream v4.20-rc1,
and was fixed in v5.3-rc1. So 4.15 was not impacted, and all 5.4
kernels already have the fix.