During an attempt to install Solaris on a real machine, I found a strange bug that caused Solaris kernel to freeze on the first boot.

This affected all post-skylake systems regardless of drive type so I originally suspected it to be related to the xHCI handoff bug with hackintosh setups, but it was actually something different.

With -v option, the last debug message shown was

root on /ramdisk:a fstype ufs

No other mentions of such bug were found on any solaris forums, but hopefully I found a similar issue on SmartOS, a solaris baed bare-metal OS: https://github.com/joyent/smartos-live/issues/727

For me, disabling both C7 states and Intel SpeedStep in BIOS setup fixed the issue, and Solaris kernel could boot up again like normal.