qemu-timer: Introduce clock reset notifier

QEMU_CLOCK_HOST is based on the system time which may jump backward in
case the admin or NTP adjusts it. RTC emulations and other device models
can suffer in this case as timers will stall for the period the clock
was tuned back.

This adds a detection mechanism that checks on every host clock readout
if the new time is before the last result. If that is the case a
notifier list is informed. Device models interested in this event can
register a notifier with the clock.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>

qemu-timer.c

@@ -150,6 +150,9 @@ struct QEMUClock {
     int enabled;
 
     QEMUTimer *warp_timer;
+
+    NotifierList reset_notifiers;
+    int64_t last;
 };
 
 struct QEMUTimer {
@@ -376,9 +379,15 @@ static QEMUTimer *active_timers[QEMU_NUM_CLOCKS];
 static QEMUClock *qemu_new_clock(int type)
 {
     QEMUClock *clock;
+
     clock = qemu_mallocz(sizeof(QEMUClock));
     clock->type = type;
     clock->enabled = 1;
+    notifier_list_init(&clock->reset_notifiers);
+    /* required to detect & report backward jumps */
+    if (type == QEMU_CLOCK_HOST) {
+        clock->last = get_clock_realtime();
+    }
     return clock;
 }
 
@@ -593,6 +602,8 @@ static void qemu_run_timers(QEMUClock *clock)
 
 int64_t qemu_get_clock_ns(QEMUClock *clock)
 {
+    int64_t now, last;
+
     switch(clock->type) {
     case QEMU_CLOCK_REALTIME:
         return get_clock();
@@ -604,10 +615,26 @@ int64_t qemu_get_clock_ns(QEMUClock *clock)
             return cpu_get_clock();
         }
     case QEMU_CLOCK_HOST:
-        return get_clock_realtime();
+        now = get_clock_realtime();
+        last = clock->last;
+        clock->last = now;
+        if (now < last) {
+            notifier_list_notify(&clock->reset_notifiers, &now);
+        }
+        return now;
     }
 }
 
+void qemu_register_clock_reset_notifier(QEMUClock *clock, Notifier *notifier)
+{
+    notifier_list_add(&clock->reset_notifiers, notifier);
+}
+
+void qemu_unregister_clock_reset_notifier(QEMUClock *clock, Notifier *notifier)
+{
+    notifier_list_remove(&clock->reset_notifiers, notifier);
+}
+
 void init_clocks(void)
 {
     rt_clock = qemu_new_clock(QEMU_CLOCK_REALTIME);

qemu-timer.h

@@ -2,6 +2,7 @@
 #define QEMU_TIMER_H
 
 #include "qemu-common.h"
+#include "notify.h"
 #include <time.h>
 #include <sys/time.h>
 
@@ -40,6 +41,10 @@ int64_t qemu_get_clock_ns(QEMUClock *clock);
 void qemu_clock_enable(QEMUClock *clock, int enabled);
 void qemu_clock_warp(QEMUClock *clock);
 
+void qemu_register_clock_reset_notifier(QEMUClock *clock, Notifier *notifier);
+void qemu_unregister_clock_reset_notifier(QEMUClock *clock,
+                                          Notifier *notifier);
+
 QEMUTimer *qemu_new_timer(QEMUClock *clock, int scale,
                           QEMUTimerCB *cb, void *opaque);
 void qemu_free_timer(QEMUTimer *ts);