linux-kernel - Re: [URGENT rfc patch 0/3] tsc clocksource bug fix

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Message-ID: <20130706105010.GV18898@dyad.programming.kicks-ass.net>
Date:	Sat, 6 Jul 2013 12:50:10 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	Frederic Weisbecker <fweisbec@...il.com>,
	Alex Shi <alex.shi@...el.com>, hpa@...ux.intel.com,
	tim.c.chen@...ux.intel.com, linux-kernel@...r.kernel.org,
	andi.kleen@...el.com, mingo@...e.hu
Subject: Re: [URGENT rfc patch 0/3] tsc clocksource bug fix

On Fri, Jul 05, 2013 at 11:50:05PM +0200, Thomas Gleixner wrote:
> On Fri, 5 Jul 2013, Peter Zijlstra wrote:
> > On Fri, Jul 05, 2013 at 05:24:09PM +0200, Thomas Gleixner wrote:
> > > See arch/x86/kernel/tsc.c
> > > 
> > > We disable the watchdog for the TSC when tsc_clocksource_reliable is
> > > set.
> > > 
> > > tsc_clocksource_reliable is set when:
> > > 
> > >  - you add tsc=reliable to the kernel command line
> > 
> > Ah, I didn't know about that one, useful.
> > 
> > >  - boot_cpu_has(X86_FEATURE_TSC_RELIABLE)
> > >  
> > >    X86_FEATURE_TSC_RELIABLE is a software flag, set by vmware and
> > >    moorsetown. So all other machines keep the watchdog enabled.
> > 
> > Right.. I knew it was enabled on my machines even though they normally
> > have usable TSC.
> 
> Yeah, but our well justified paranoia still prevents us from trusting
> these CPU flags. Maybe some day BIOS is going to be replaced by
> something useful. You know: Hope springs eternal....

Oh quite agreed. Its just that at several times I've wanted to disable the
thing. Now I know you can do using the kernel cmdline. Previously I had to
wreck code -- not that much harder really :-)
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