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vmx: reduce cpu load by eliminating nop-loop

Sacrifice some of the sub-millisecond timer precision in favor of less
cpu load when the timer is about to be kicked a bit early.  Result is
visible *especially* when the guest idling.

Timer proc *still* has to send to the channel (in order to kick PIT
and RTC logic), which takes time, and compensates a bit for possibly
early runs.

--- a/sys/src/cmd/vmx/vmx.c

+++ b/sys/src/cmd/vmx/vmx.c

@@ -423,12 +423,13 @@

 static void

 sleeperproc(void *)

 {

-	vlong then, now;

+	vlong then, now, dt, adj;

 	timerid = threadid();

 	timerevent = nanosec() + SleeperPoll;

 	unlock(&timerlock);

 	threadsetname("sleeper");

+	adj = 0;

 	for(;;){

 		lock(&timerlock);

 		then = timerevent;

@@ -435,12 +436,15 @@

 		now = nanosec();

 		if(then <= now) timerevent = now + SleeperPoll;

 		unlock(&timerlock);

-		if(then - now >= MinSleep){

-			sleep((then - now) / MSEC);

-			continue;

+		if(then > now){

+			dt = then - now;

+			if(dt+adj >= MinSleep){

+				sleep((dt + adj) / MSEC);

+				continue;

+			}

+			adj = dt;

 		}

-		while(nanosec() < then)

-			;

+

 		sendul(sleepch, 0);

 	}

 }