When setting up interrupt remapping for legacy PCI(-X) devices, including PCI(-X) bridges, a lookup of the upstream bridge is required. This lookup, itself involving acquiring of a lock, is done in a context where acquiring that lock is unsafe. This can lead to a deadlock.
https://xenbits.xenproject.org/xsa/advisory-467.html
http://xenbits.xen.org/xsa/advisory-467.html
http://www.openwall.com/lists/oss-security/2025/02/28/1