linux-kernel - Re: [PATCH] x86 (64): make calibrate_APIC

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Message-ID: <4889968E.6020000@fujitsu-siemens.com>
Date:	Fri, 25 Jul 2008 11:02:06 +0200
From:	Martin Wilck <martin.wilck@...itsu-siemens.com>
To:	Martin Wilck <martin.wilck@...itsu-siemens.com>
CC:	Cyrill Gorcunov <gorcunov@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	"Wichert, Gerhard" <Gerhard.Wichert@...itsu-siemens.com>,
	"Maciej W. Rozycki" <macro@...ux-mips.org>
Subject: Re: [PATCH] x86 (64): make calibrate_APIC_clock() SMI-safe (take
 2)

I wrote:

> This patch fixes this by two separate measures:
>   a) make sure that no significant interruption occurs between APIC and
>      TSC reads
>   b) make sure that the measurement loop isn't significantly longer
>      than originally intended.

Here is a new, simplified version of our patch that only uses measure a).
We verified that this is sufficient for accurate calibration.

If we fail to determine the start or end time of the calibration 
correctly 10 times in a row, we will print a critical error message and 
go on. One might as well argue that this should cause a kernel panic (it 
is impossible to run on the CPU for only a few cycles without being 
interrupted by an SMI!), but Cyrill probably won't agree.

Martin

-- 
Martin Wilck
PRIMERGY System Software Engineer
FSC IP ESP DEV 6

Fujitsu Siemens Computers GmbH
Heinz-Nixdorf-Ring 1
33106 Paderborn
Germany

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View attachment "calibrate_APIC_clock-4.diff" of type "text/x-patch" (2873 bytes)