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@fujitsu-siemens.com>
Signed-off-by: Gerhard Wichert <gerhard.wichert@fujitsu-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);
 	}