Messages in this thread | | | From | Andy Lutomirski <> | Date | Fri, 20 Mar 2020 16:57:03 -0700 | Subject | Re: [PATCH v2 2/2] x86/delay: Introduce TPAUSE delay |
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On Fri, Mar 20, 2020 at 4:23 PM Thomas Gleixner <tglx@linutronix.de> wrote: > > Andy Lutomirski <luto@kernel.org> writes: > > > On Fri, Mar 20, 2020 at 3:00 AM Thomas Gleixner <tglx@linutronix.de> wrote: > >> > >> Andy Lutomirski <luto@kernel.org> writes: > >> > On Thu, Mar 19, 2020 at 9:13 PM Kyung Min Park <kyung.min.park@intel.com> 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 :)
> > Thanks, > > tglx > > > > > > > >
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