A local attacker (e.g., an unprivileged guest) with access to a passed-through legacy PCI(-X) device on a system using Intel VT-d can trigger a deadlock in the Xen hypervisor. This can result in a denial of service (DoS) that affects the entire host. The issue arises due to unsafe locking during interrupt remapping setup for certain devices. The vulnerability impacts Xen versions 4.0 and later on systems with Intel IOMMU (VT-d) hardware. Systems using AMD or non-x86 hardware are not affected. Avoiding pass through of the affected device types mitigates the issue.