re: What to do about "WARNING: negative runtime; monotonic clock has gone backwards"

[matthew green <mrg%eterna.com.au@localhost>]

one problem i've seen in kern_tc.c when the timecounter returns
a smaller value is that tc_delta() ends up returning a very large
(underflowed) value, and that makes the consumers of it do a very
wrong thing.  eg, -2 becomes 2^32-2, and then eg in binuptime:

477                 bintime_addx(bt, th->th_scale * tc_delta(th));

or in tc_windup():

933         delta = tc_delta(th);
938         th->th_offset_count += delta;
939         bintime_addx(&th->th_offset, th->th_scale * delta);

i "fixed" the time goes backwards on sparc issue a few years ago
with this change, which avoids the above issue:

   http://mail-index.netbsd.org/source-changes/2018/01/12/msg091064.html

but i really think that the way tc_delta() can underflow is a
bad problem we should fix properly, i just wasn't sure of the
right way to do it.


.mrg.