Close a race when making the CPU idle under pHyp. If an interrupt occurs

between the beginning of the idle function and actually going idle, the
CPU could go to sleep with pending work.

MFC after:	1 month

sys/powerpc/pseries/platform_chrp.c

@@ -34,6 +34,7 @@ __FBSDID("$FreeBSD$");
 #include <sys/bus.h>
 #include <sys/pcpu.h>
 #include <sys/proc.h>
+#include <sys/sched.h>
 #include <sys/smp.h>
 #include <vm/vm.h>
 #include <vm/pmap.h>
@@ -492,7 +493,18 @@ chrp_reset(platform_t platform)
 static void
 phyp_cpu_idle(sbintime_t sbt)
 {
-	phyp_hcall(H_CEDE);
+	register_t msr;
+
+	msr = mfmsr();
+
+	mtmsr(msr & ~PSL_EE);
+	if (sched_runnable()) {
+		mtmsr(msr);
+		return;
+	}
+
+	phyp_hcall(H_CEDE); /* Re-enables interrupts internally */
+	mtmsr(msr);
 }
 
 static void