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guesscpuhz: add sanity checks for virtualbox (import from sources)

--- a/sys/src/9/pc/i8253.c

+++ b/sys/src/9/pc/i8253.c

@@ -165,7 +165,7 @@

 	cpufreq = (vlong)loops*((aalcycles*2*Freq)/x);

 	m->loopconst = (cpufreq/1000)/aalcycles;	/* AAM+LOOP's for 1 ms */

-	if(m->havetsc){

+	if(m->havetsc && a != b){  /* a == b means virtualbox has confused us */

 		/* counter goes up by 2*Freq */

 		b = (b-a)<<1;

 		b *= Freq;

@@ -185,6 +185,9 @@

 		m->cpuhz = cpufreq;

 	}

+	/* don't divide by zero in trap.c */

+	if(m->cpumhz == 0)

+		panic("guesscpuhz: zero m->cpumhz");

 	i8253.hz = Freq<<Tickshift;

 }

-- 

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