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From: Mikael Pettersson <mikpe@csd.uu.se>
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Date: Fri, 15 Nov 2002 10:50:28 +0100
To: Corey Minyard <cminyard@mvista.com>
Cc: "Heater, Daniel (IndSys, GEFanuc, VMIC)" <Daniel.Heater@gefanuc.com>,
       "'Zwane Mwaikambo'" <zwane@holomorphy.com>,
       John Levon <levon@movementarian.org>,
       Dipankar Sarma <dipankar@gamebox.net>, linux-kernel@vger.kernel.org
Subject: Re: NMI handling rework for x86
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Corey Minyard writes:
 > diff -urN linux.orig/arch/i386/kernel/nmi_watchdog.c linux/arch/i386/kernel/nmi_watchdog.c
 > --- linux.orig/arch/i386/kernel/nmi_watchdog.c	Thu Oct 24 19:56:54 2002
 > +++ linux/arch/i386/kernel/nmi_watchdog.c	Thu Oct 24 20:54:19 2002
...
 > +static int k7_watchdog_reset(int handled)
 > +{
 > +	unsigned int low, high;
 > +	int          source;
 > +
 > +	rdmsr(MSR_K7_PERFCTR0, low, high);
 > +	source = (low & (1 << 31)) == 0;
 > +	if (source)
 > +		wrmsr(MSR_K7_PERFCTR0, -(cpu_khz/nmi_hz*1000), -1);
 > +	return source;
 > +}

and similar code in p6 and p4 watchdog_reset.

- Why are you reading the perfctrs with RDMSR instead of RDPMC?
  RDMSR is noticeably slower on most post-P5 CPUs.
- "(low & (1 << 31)) == 0" looks like a convoluted and inefficient
  way of computing "(int)low >= 0".
- The p6 and k7 watchdog_reset procedures are identicial, except
  for the actual MSR number used. The original nmi.c makes the
  number a parameter and shares the code, causing less code bloat.

/Mikael
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