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From:   Andy Lutomirski <luto@kernel.org>
Date:   Fri, 20 Mar 2020 14:51:08 -0700
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Subject: Re: [PATCH v2 2/2] x86/delay: Introduce TPAUSE delay
To:     Thomas Gleixner <tglx@linutronix.de>
Cc:     Andy Lutomirski <luto@kernel.org>,
        Kyung Min Park <kyung.min.park@intel.com>,
        X86 ML <x86@kernel.org>, LKML <linux-kernel@vger.kernel.org>,
        Ingo Molnar <mingo@redhat.com>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Greg KH <gregkh@linuxfoundation.org>,
        Andi Kleen <ak@linux.intel.com>,
        Tony Luck <tony.luck@intel.com>,
        "Raj, Ashok" <ashok.raj@intel.com>,
        "Ravi V. Shankar" <ravi.v.shankar@intel.com>,
        Fenghua Yu <fenghua.yu@intel.com>
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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.