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

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Date:	Fri, 25 Jul 2008 17:48:31 +0400
From:	Cyrill Gorcunov <gorcunov@...il.com>
To:	Martin Wilck <martin.wilck@...itsu-siemens.com>
Cc:	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)

[Martin Wilck - Fri, Jul 25, 2008 at 03:38:47PM +0200]
> I wrote:
>
>> I have to say that my simplified patch failed to do the calibration
>> correctly on our test system (the original patch worked well). Please
>> stay tuned, we are investigating this currently.
>
> Please forget that. It was observed on an old "enterprise" kernel with  
> which we are currently testing the backported patch with, and it was due  
> to the fact that the initial counter value on that kernel was divided by  
> APIC_DIVISOR (=16). The resulting initial counter value was too low in a  
> "SMI flood" case, the counter could overlap. APIC_DIVISOR is no longer  
> used in the current kernel.
>
> Martin
>
> -- 

Martin, if I understood you right - this means your patch is not
needed? Actually on 64bit mode APIC_DIVISOR is a bit hidden in
__setup_APIC_LVTT - you may see it as APIC_TDR_DIV_16 while setting
up divisor register. I was proposing patch for that but it leaded
to potetntial overflow (thanks Ingo for catching) so we leave it as
is. Maybe I miss something?

		- Cyrill -
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