RFC: Simplfying hyperthreading distinctions

[Rui Paulo]

> On 6 Mar 2015, at 12:44, John Baldwin <jhb at FreeBSD.org> wrote:
> 
> 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.

Yes, this is annoying.

>  I propose the 
> following changes to simplify things a bit:
> 
>  1) Call both "old" and "new" hyperthreading HTT in dmesg.

Yes.

>  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.

I think that's ok given 3).

>  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.

Sounds good.

Thanks,
--
Rui Paulo