linux-kernel - RE: [PATCH] x86: Export tsc related information in sysfs

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Date:	Mon, 24 May 2010 17:01:06 -0700 (PDT)
From:	Dan Magenheimer <dan.magenheimer@...cle.com>
To:	john stultz <johnstul@...ibm.com>
Cc:	"H. Peter Anvin" <hpa@...or.com>,
	Brian Bloniarz <bmb@...enacr.com>, Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	Andi Kleen <andi@...stfloor.org>,
	Arjan van de Ven <arjan@...radead.org>,
	Venkatesh Pallipadi <venki@...gle.com>, chris.mason@...cle.com,
	linux-kernel@...r.kernel.org
Subject: RE: [PATCH] x86: Export tsc related information in sysfs

> Sorry, this is sort of mixing points
> Again, this is mixing the discussion.

Oops, sorry, missed that. :-}
 
> Maybe something like a tsc_long_calibration=1 option would allow for
> this?

Sounds good to me.  If it's non-obvious what value to choose for
the new calibration, maybe specifying it in MS (per MAX_QUICK_PIT_MS
in arch/x86/kernel/tsc.c) would be nice.

> However, I really do like the idea of pulling the stamped value from
> the MSR and if its close to what we quickly calibrated, use that.

On a quick sample of two machines looking at the TSC calibration
done by Xen (which exposes the equivalent of tsc_khz), it appears
that the stamped value is different from the calibration by about
1000ppm.  YMMV.
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