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

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Date:	Thu, 24 Jul 2008 15:16:31 +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() smp-safe

[Martin Wilck - Thu, Jul 24, 2008 at 12:47:56PM +0200]
> Hi Thomas and Peter, hi everyone,
>
> Asynchrounous events (e.g.SMIs) which occur during the APIC timer  
> calibration can cause timer miscalibrations, sometimes by large amounts.
>
> 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.
>
> I am sorry, due to a misconfiguration of our SMTP server I need to send  
> the patch as attachment.
>
> Martin
>
> -- 
> Martin Wilck
> PRIMERGY System Software Engineer
> FSC IP ESP DEV 6
>
> Fujitsu Siemens Computers GmbH
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> 33106 Paderborn
> Germany
>
> Tel:			++49 5251 8 15113
> Fax:			++49 5251 8 20209
> Email:			mailto:martin.wilck@...itsu-siemens.com
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| Patch: make calibrate_APIC_clock() SMI-safe
| 
| Asynchrounous events (e.g.SMIs) which occur during the APIC timer calibration
| can cause timer miscalibrations, sometimes by large amounts. 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.
| 
| Signed-off-by: Martin Wilck <martin.wilck@...itsu-siemens.com>
| Signed-off-by: Gerhard Wichert <gerhard.wichert@...itsu-siemens.com>
| 
| --- linux-2.6.26/arch/x86/kernel/apic_64.c	2008-07-13 23:51:29.000000000 +0200
| +++ linux-2.6.26/arch/x86/kernel/apic_64.c.new	2008-07-24 11:41:24.000000000 +0200
| @@ -314,6 +314,19 @@ static void setup_APIC_timer(void)
|  
|  #define TICK_COUNT 100000000
|  
| +#define MAX_DIFFERENCE 1000UL
| +static inline void __read_tsc_and_apic(unsigned long *tsc, unsigned *apic)
| +{
| +	unsigned long tsc0, tsc1, diff;
| +	do {
| +		rdtscll(tsc0);
| +		*apic = apic_read(APIC_TMCCT);
| +		rdtscll(tsc1);
| +		diff = tsc1 - tsc0;
| +	} while (diff > MAX_DIFFERENCE);
| +	*tsc = tsc0 + (diff >> 1);
| +}
| +
|  static void __init calibrate_APIC_clock(void)
|  {
|  	unsigned apic, apic_start;
| @@ -329,25 +342,37 @@ static void __init calibrate_APIC_clock(
|  	 *
|  	 * No interrupt enable !
|  	 */
| +smi_occured:
|  	__setup_APIC_LVTT(250000000, 0, 0);
|  
| -	apic_start = apic_read(APIC_TMCCT);
|  #ifdef CONFIG_X86_PM_TIMER
|  	if (apic_calibrate_pmtmr && pmtmr_ioport) {
| +		apic_start = apic_read(APIC_TMCCT);
|  		pmtimer_wait(5000);  /* 5ms wait */
|  		apic = apic_read(APIC_TMCCT);
|  		result = (apic_start - apic) * 1000L / 5;
|  	} else
|  #endif
|  	{
| -		rdtscll(tsc_start);
| +		__read_tsc_and_apic(&tsc_start, &apic_start);
|  
|  		do {
| -			apic = apic_read(APIC_TMCCT);
| -			rdtscll(tsc);
| +			__read_tsc_and_apic(&tsc, &apic);
|  		} while ((tsc - tsc_start) < TICK_COUNT &&
|  				(apic_start - apic) < TICK_COUNT);
|  
| +		/*
| +		 * If this takes significantly longer than TICK_COUNT,
| +		 * some interruption must have occured - retry.
| +		 */
| +		if ((tsc - tsc_start) > (TICK_COUNT + TICK_COUNT/1000) ||
| +		    (apic_start - apic) > (TICK_COUNT + TICK_COUNT/1000)) {
| +			printk(KERN_ERR
| +			       "calibrate_APIC_clock: SMI occured? %lx %x",
| +			       tsc - tsc_start, apic_start - apic);
| +			goto smi_occured;
| +		}
| +
|  		result = (apic_start - apic) * 1000L * tsc_khz /
|  					(tsc - tsc_start);
|  	}

Hi, but what if SMI flood happens? We could stuck here forever, don't we?

		- Cyrill -
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