[Qemu-devel] [PATCH v2] Restores record/replay behavior related to special virtual clock processing for timers used in external subsystems.

[Artem Pisarenko]

Adds EXTERNAL attribute definition to qemu timers subsystem and assigns it to 
virtual clock timers, used in slirp (ICMP IPv6) and ui (key queue).
Virtual clock processing in rr mode reimplemented using this attribute.

Fixes: 87f4fe7653baf55b5c2f2753fe6003f473c07342
Fixes: 775a412bf83f6bc0c5c02091ee06cf649b34c593
Fixes: 9888091404a702d7ec79d51b088d994b9fc121bd
Signed-off-by: Artem Pisarenko <address@hidden>
---

Notes:
    v2: fixes race condition and reimplements synchronization between 
checkpointing and timers processing in qemu-timer.c
    
    qemu-timer.c:
    It has one more difference from original behavior, which I'm not sure about.
    If there are no timers will be processed for virtual clock (i.e. timer list 
is empty), then checkpointing will be skipped, although it looks fine for me. 
Is such scenario ever possible?

 include/qemu/timer.h | 10 ++++++++--
 slirp/ip6_icmp.c     |  4 +++-
 ui/input.c           |  5 +++--
 util/qemu-timer.c    | 50 +++++++++++++++++++++++++++++++++++++++++++-------
 4 files changed, 57 insertions(+), 12 deletions(-)

diff --git a/include/qemu/timer.h b/include/qemu/timer.h
index 031e3a1..53bfba5 100644
--- a/include/qemu/timer.h
+++ b/include/qemu/timer.h
@@ -65,11 +65,17 @@ typedef enum {
  * instead each attribute in bit set accessed with QEMU_TIMER_ATTR(id) macro,
  * where 'id' is a unique part of attribute identifier.
  *
- * No attributes defined currently.
+ * The following attributes are available:
+ *
+ * QEMU_TIMER_ATTR(EXTERNAL): drives external subsystem
+ *
+ * Timers with this attribute do not recorded in rr mode, therefore it could be
+ * used for the subsystems that operate outside the guest core. Applicable only
+ * with virtual clock type.
  */
 
 typedef enum {
-    /* none */
+    QEMU_TIMER_ATTRBIT_EXTERNAL,
 } QEMUTimerAttrBit;
 
 #define QEMU_TIMER_ATTR(id) (1 << QEMU_TIMER_ATTRBIT_ ## id)
diff --git a/slirp/ip6_icmp.c b/slirp/ip6_icmp.c
index ee333d0..7c08433 100644
--- a/slirp/ip6_icmp.c
+++ b/slirp/ip6_icmp.c
@@ -27,7 +27,9 @@ void icmp6_init(Slirp *slirp)
         return;
     }
 
-    slirp->ra_timer = timer_new_ms(QEMU_CLOCK_VIRTUAL, ra_timer_handler, 
slirp);
+    slirp->ra_timer = timer_new_a(QEMU_CLOCK_VIRTUAL, SCALE_MS,
+                                  QEMU_TIMER_ATTR(EXTERNAL),
+                                  ra_timer_handler, slirp);
     timer_mod(slirp->ra_timer,
               qemu_clock_get_ms(QEMU_CLOCK_VIRTUAL) + NDP_Interval);
 }
diff --git a/ui/input.c b/ui/input.c
index 51b1019..6279187 100644
--- a/ui/input.c
+++ b/ui/input.c
@@ -448,8 +448,9 @@ void qemu_input_event_send_key_delay(uint32_t delay_ms)
     }
 
     if (!kbd_timer) {
-        kbd_timer = timer_new_ms(QEMU_CLOCK_VIRTUAL, qemu_input_queue_process,
-                                 &kbd_queue);
+        kbd_timer = timer_new_a(QEMU_CLOCK_VIRTUAL, SCALE_MS,
+                                QEMU_TIMER_ATTR(EXTERNAL),
+                                qemu_input_queue_process, &kbd_queue);
     }
     if (queue_count < queue_limit) {
         qemu_input_queue_delay(&kbd_queue, kbd_timer,
diff --git a/util/qemu-timer.c b/util/qemu-timer.c
index 29d8e39..7731dd9 100644
--- a/util/qemu-timer.c
+++ b/util/qemu-timer.c
@@ -490,6 +490,7 @@ bool timerlist_run_timers(QEMUTimerList *timer_list)
     bool progress = false;
     QEMUTimerCB *cb;
     void *opaque;
+    bool need_replay_checkpoint = false;
 
     if (!atomic_read(&timer_list->active_timers)) {
         return false;
@@ -505,8 +506,15 @@ bool timerlist_run_timers(QEMUTimerList *timer_list)
         break;
     default:
     case QEMU_CLOCK_VIRTUAL:
-        if (!replay_checkpoint(CHECKPOINT_CLOCK_VIRTUAL)) {
-            goto out;
+        if (replay_mode != REPLAY_MODE_NONE) {
+            /* Checkpoint for virtual clock is redundant in cases where
+             * it's being triggered with only non-EXTERNAL timers, because
+             * these timers don't change guest state directly.
+             * Since it has conditional dependence on specific timers, it is
+             * subject to race conditions and requires special handling.
+             * See below.
+             */
+            need_replay_checkpoint = true;
         }
         break;
     case QEMU_CLOCK_HOST:
@@ -521,13 +529,38 @@ bool timerlist_run_timers(QEMUTimerList *timer_list)
         break;
     }
 
+    /*
+     * Extract expired timers from active timers list and and process them.
+     *
+     * In rr mode we need "filtered" checkpointing for virtual clock.
+     * Checkpoint must be replayed before any non-EXTERNAL timer has been
+     * processed and only one time (virtual clock value stays same). But these
+     * timers may appear in the timers list while it being processed, so this
+     * must be checked until we finally decide that "no timers left - we are
+     * done".
+     */
     current_time = qemu_clock_get_ns(timer_list->clock->type);
-    for(;;) {
-        qemu_mutex_lock(&timer_list->active_timers_lock);
-        ts = timer_list->active_timers;
+    qemu_mutex_lock(&timer_list->active_timers_lock);
+    while ((ts = timer_list->active_timers)) {
         if (!timer_expired_ns(ts, current_time)) {
+            /* No expired timers left.
+             * (If rr checkpoint was needed, it either already handled,
+             *  or may be skipped.) */
+            break;
+        }
+        if (need_replay_checkpoint
+                && !(ts->attributes & QEMU_TIMER_ATTR(EXTERNAL))) {
+            /* once we got here, checkpoint clock only once */
+            need_replay_checkpoint = false;
             qemu_mutex_unlock(&timer_list->active_timers_lock);
-            break;
+            if (!replay_checkpoint(CHECKPOINT_CLOCK_VIRTUAL)) {
+                goto out;
+            }
+            qemu_mutex_lock(&timer_list->active_timers_lock);
+            /* it's better to start over again,
+             * just in case if timer list was modified
+             */
+            continue;
         }
 
         /* remove timer from the list before calling the callback */
@@ -536,12 +569,15 @@ bool timerlist_run_timers(QEMUTimerList *timer_list)
         ts->expire_time = -1;
         cb = ts->cb;
         opaque = ts->opaque;
-        qemu_mutex_unlock(&timer_list->active_timers_lock);
 
         /* run the callback (the timer list can be modified) */
+        qemu_mutex_unlock(&timer_list->active_timers_lock);
         cb(opaque);
+        qemu_mutex_lock(&timer_list->active_timers_lock);
+
         progress = true;
     }
+    qemu_mutex_unlock(&timer_list->active_timers_lock);
 
 out:
     qemu_event_set(&timer_list->timers_done_ev);
-- 
2.7.4