AMD iommu: Difference between setting iommu=pt and not setting this option

[Andreas Hartmann <andihartmann@xxxxxxxxxxxxxxx>]

Hello,

please, could somebody explain the difference in behaviour between
setting the boot option iommu=pt and not setting it?

I can see the following differences:

w/ iommu=pt:

[    0.832946] AMD-Vi: Enabling IOMMU at 0000:00:00.2 cap 0x40
[    0.883983] AMD-Vi: Initialized for Passthrough Mode


w/o iommu=pt:

[    0.834905] AMD-Vi: Enabling IOMMU at 0000:00:00.2 cap 0x40
[    0.894172] AMD-Vi: Lazy IO/TLB flushing enabled



In both cases, I can pass through a PCIe device but not a PCI device (->
no problem with IRQ-sharing - the PCI device has its own exclusive IRQ).

If I use iommu=pt, the passed PCIe device is broken after the VM has
been shutdown and started again.



Thank you,
kind regards,
Andreas
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