Code for recording timestamps of events, especially function entries/exits.
This is a very primitive system, intended for use in measuring performance
during the early system boot, before more sophisticated tools like DTrace
or infrastructure like kernel memory allocation and mutexes are available.

Because this code records pointers to strings rather than copying strings
(in order to keep the memory usage more manageable), if a kernel module is
unloaded after logging an event, Bad Things can happen. Users are advised
to not do that.

Since cycle counts from the early kernel boot are used as an initial entropy
source, publishing this information to userland could result in inadequate
entropy being kept private to the kernel RNG. Users are advised to not
enable this on systems with untrusted users.

Discussed on: freebsd-current