RFC: Simplfying hyperthreading distinctions

[John Baldwin]

Currently we go out of our way a bit to distinguish Pentium4-era 
hyperthreading from more recent ("modern") hyperthreading.  I suspect that 
this distinction probably results in confusion more than anything else.  
Intel's documentation does not make near as broad a distinction as far as I 
can tell.  Both types of SMT are called hyperthreading in the SDM for example.  
However, we have the astonishing behavior that 
'machdep.hyperthreading_allowed' only affects "old" hyperthreads, but not 
"new" ones.  We also try to be overly cute in our dmesg output by using HTT 
for "old" hyperthreading, and SMT for "new" hyperthreading.  I propose the 
following changes to simplify things a bit:

  1) Call both "old" and "new" hyperthreading HTT in dmesg.

  2) Change machdep.hyperthreading_allowed to apply to both new and old HTT.
     However, doing this means a POLA violation in that we would now disable
     modern HTT by default.  Balanced against re-enabling "old" HTT by default
     on an increasingly-shrinking pool of old hardware, I think the better
     approach here would be to also change the default to allow HTT.

  3) Possibly add a different knob (or change the behavior of
     machdep.hyperthreading_allowed) to still bring up hyperthreads, but leave
     them out of the default cpuset (set 1).  This would allow those threads
     to be re-enabled dynamically at runtime by adjusting the mask on set 1.
     The original htt settings back when 'hyperthreading_allowed' was
     introduced actually permitted this via by adjusting 'machdep.hlt_cpus' at
     runtime.

What do people think?

-- 
John Baldwin