TBI is a very useful feature of ARMv8 that allows the top byte of pointers to be used as a tag. This is useful for various JIT compilers, HWASAN etc. This has been enabled on Linux since 2013, but not on FreeBSD yet. (upd: also on Linux it's per-process toggleable via procctl or something now?)
Linux clears the bits on EL0 data abort, so I've done the same (??) here — not sure about that actually.