pciback: question about the permissive flag

[Joanna Rutkowska <joanna@invisiblethingslab.com>]

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I'm trying to understand the purpose of the permissive flag in the Xen
pciback driver. The comments in the code suggest that setting
permissive=1 is "potentially unsafe", and I've been wondering why?

My thinking goes this way -- we either:

1) have IOMMU/VT-d in the system, and use it to isolate the device
assigned to a DomU, in which case allowing the DomU to fully control the
assigned device's config space should not be a problem because VT-d
should do its job (we hope at least ;),

or

2) we don't have IOMMU/VT-d, in which case assigning a device to
anything other than Dom0 is simply insecure, no matter if we try to
restrict access to config space (but still allow DMA engine to be
programmed by DomU) or not.

So, what am I missing here?

joanna.


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