To avoid an erratum in early hardware, the Xen AMD IOMMU code by default chooses to use a single interrupt remapping table for the whole system.  This sharing implies that any guest with a passed through PCI device that is bus mastering capable can inject interrupts into other guests, including domain 0.

Furthermore, regardless of whether a shared interrupt remapping table is in use, old entries are not always cleared, providing opportunities (which accumulate over time) for guests to inject interrupts into other guests, again including domain 0. 

In a typical Xen system many devices are owned by domain 0 or driver domains, leaving them vulnerable to such an attack. Such a DoS is likely to have an impact on other guests running in the system. 

A malicious domain which is given access to a physical PCI device can mount a denial of service attack affecting the whole system.

Acknowledgements:

Red Hat would like to thank the Xen project for reporting this issue.