stop and restart kernel event timers in the suspend / resume cycle

I have a system that is very unstable after resuming from suspend-to-RAM
but only if HPET is used as the event timer.  The theory is that SMM
code / firmware could be enabling HPET for its own uses and unexpected
interrupts cause a trouble for it.  Originally I wanted to solve the
problem in hpet_suspend() method, but that was insufficient as the event
timer could get reprogrammed again.

So, it's better, for my case and in general, to stop the event timer(s)
before entering the hardware suspend.

MFC after:	4 weeks
Differential Revision: https://reviews.freebsd.org/D15413

sys/dev/acpica/acpi.c

@@ -2958,6 +2958,7 @@ acpi_EnterSleepState(struct acpi_softc *sc, int state)
     if (sc->acpi_sleep_delay > 0)
 	DELAY(sc->acpi_sleep_delay * 1000000);
 
+    suspendclock();
     intr = intr_disable();
     if (state != ACPI_STATE_S1) {
 	sleep_result = acpi_sleep_machdep(sc, state);
@@ -3028,6 +3029,8 @@ acpi_EnterSleepState(struct acpi_softc *sc, int state)
      * process.  This handles both the error and success cases.
      */
 backout:
+    if (slp_state >= ACPI_SS_SLP_PREP)
+	resumeclock();
     if (slp_state >= ACPI_SS_GPE_SET) {
 	acpi_wake_prep_walk(state);
 	sc->acpi_sstate = ACPI_STATE_S0;

sys/kern/kern_clocksource.c

@@ -698,6 +698,22 @@ cpu_initclocks_ap(void)
 	spinlock_exit();
 }
 
+void
+suspendclock(void)
+{
+	ET_LOCK();
+	configtimer(0);
+	ET_UNLOCK();
+}
+
+void
+resumeclock(void)
+{
+	ET_LOCK();
+	configtimer(1);
+	ET_UNLOCK();
+}
+
 /*
  * Switch to profiling clock rates.
  */

sys/sys/systm.h

@@ -333,6 +333,8 @@ void	startprofclock(struct proc *);
 void	stopprofclock(struct proc *);
 void	cpu_startprofclock(void);
 void	cpu_stopprofclock(void);
+void	suspendclock(void);
+void	resumeclock(void);
 sbintime_t 	cpu_idleclock(void);
 void	cpu_activeclock(void);
 void	cpu_new_callout(int cpu, sbintime_t bt, sbintime_t bt_opt);