Apparent lock up (excessive delay, no interupt handling) during boot

Occurs for both basic and advanced install, and reports of similar behavior on an installed XenServer.
Xen kernel finishes loading, new Dom0 kernel begins to load but hangs.
Last observed log entry prior to the block is "PCI bridge to [bus 09-0a]", with the next device (08) being an Adaptec RAID card that exposes a VPD endpoint but doesn't provide any content.
This should not block the system's boot, and other Linux implementations do not block here.

System currently has Xenserver 6.5 installed and functional, previously ran CentOS7 with many months of uptime