Ensure the idle thread's loop services interrupts in a timely way when

using the ACPI C1/mwait sleep method.

Previously, the mwait instruction would return when an interrupt was
pending; however, the idle loop did not actually enable interrupts when
this occurred. This led to a situation where the idle loop could quickly
spin through the C1/mwait sleep method a number of times when an interrupt
was pending. (Eventually, the situation corrected itself when something
other than an interrupt triggered the idle loop to either enable interrupts
or schedule another thread.)

Reviewed by:	kib, imp (earlier version)
Input from:	jhb
MFC after:	1 week
Sponsored by:	Netflix

sys/dev/acpica/acpi_cpu.c

@@ -1158,6 +1158,9 @@ acpi_cpu_idle(sbintime_t sbt)
 	end_time = ((cpu_ticks() - cputicks) << 20) / cpu_tickrate();
 	if (curthread->td_critnest == 0)
 		end_time = min(end_time, 500000 / hz);
+	/* acpi_cpu_c1() returns with interrupts enabled. */
+	if (cx_next->do_mwait)
+	    ACPI_ENABLE_IRQS();
 	sc->cpu_prev_sleep = (sc->cpu_prev_sleep * 3 + end_time) / 4;
 	return;
     }

sys/x86/x86/cpu_machdep.c

@@ -129,6 +129,14 @@ acpi_cpu_c1(void)
 	__asm __volatile("sti; hlt");
 }
 
+/*
+ * Use mwait to pause execution while waiting for an interrupt or
+ * another thread to signal that there is more work.
+ *
+ * NOTE: Interrupts will cause a wakeup; however, this function does
+ * not enable interrupt handling. The caller is responsible to enable
+ * interrupts.
+ */
 void
 acpi_cpu_idle_mwait(uint32_t mwait_hint)
 {