In the loader menu, drop into:
2. [Esc]ape to loader prompt
then
boot -kv -m verbose -B nmi=kmdb
When it hangs, trigger an NMI. (For example, virsh inject-nmi smartos).
Back on the console, you should have dropped into KMDB, and can get a backtrace:
[0]> $C
fffffffffbc13e70 kmdb_enter+0xb()
fffffffffbc13ea0 debug_enter+0x59(fffffffff794d6d3)
fffffffffbc13ed0 apix`apic_nmi_intr+0x9e(0, fffffffffbc13f10)
fffffffffbc13f00 av_dispatch_nmivect+0x34(fffffffffbc13f10)
fffffffffbc13f10 nmiint+0x152()
...
Other commands such as ::stacks and ::walk cpu | ::cpustack might be useful at this point.