RE: [PATCH v2 2/2] x86/delay: Introduce TPAUSE delay

[Park, Kyung Min]

Hi Andy/Thomas,

 On Fri, Mar 20, 2020 at 4:23 PM Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote:
> >
> > Andy Lutomirski <luto@xxxxxxxxxx> writes:
> >
> > > On Fri, Mar 20, 2020 at 3:00 AM Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> wrote:
> > >>
> > >> Andy Lutomirski <luto@xxxxxxxxxx> writes:
> > >> > On Thu, Mar 19, 2020 at 9:13 PM Kyung Min Park
> <kyung.min.park@xxxxxxxxx> wrote:
> > >> >>  void use_tsc_delay(void)
> > >> >>  {
> > >> >> -       if (delay_fn == delay_loop)
> > >> >> +       if (static_cpu_has(X86_FEATURE_WAITPKG)) {
> > >> >> +               delay_halt_fn = delay_halt_tpause;
> > >> >> +               delay_fn = delay_halt;
> > >> >> +       } else if (delay_fn == delay_loop) {
> > >> >>                 delay_fn = delay_tsc;
> > >> >> +       }
> > >> >>  }
> > >> >
> > >> > This is an odd way to dispatch: you're using static_cpu_has(),
> > >> > but you're using it once to populate a function pointer.  Why not
> > >> > just put the static_cpu_has() directly into delay_halt() and
> > >> > open-code the three variants?
> > >>
> > >> Two: mwaitx and tpause.
> > >
> > > I was imagining there would also be a variant for systems with neither
> feature.
> >
> > Oh I see, you want to get rid of both function pointers. That's tricky.
> >
> > The boot time function is delay_loop() which is using the magic (1 <<
> > 12) boot time value until calibration in one way or the other happens
> > and something calls use_tsc_delay() or use_mwaitx_delay(). Yes, that's
> > all horrible but X86_FEATURE_TSC is unusable for this.
> >
> > Let me think about it.
> 
> This is definitely not worth overoptimizing.  It's a *delay* function
> -- the retpoline isn't going to kill us :)

Since the use_tsc_delay() is used just once in __init tsc_init(), 
how about adding "__init" to the use_tsc_delay() and keep these function pointers?