[Qemu-devel] [PATCH 2/3] Fix time drift problem under high load when PIT is in use.

[Gleb Natapov]

Count the number of interrupts that was lost due to interrupt coalescing
and re-inject them back when possible. This fixes time drift problem when
pit is used as a time source.

Signed-off-by: Gleb Natapov <address@hidden>
---

 hw/i8254.c |   20 +++++++++++++++++++-
 1 files changed, 19 insertions(+), 1 deletions(-)

diff --git a/hw/i8254.c b/hw/i8254.c
index 4813b03..c4f0f46 100644
--- a/hw/i8254.c
+++ b/hw/i8254.c
@@ -61,6 +61,8 @@ static PITState pit_state;
 
 static void pit_irq_timer_update(PITChannelState *s, int64_t current_time);
 
+static uint32_t pit_irq_coalesced;
+
 static int pit_get_count(PITChannelState *s)
 {
     uint64_t d;
@@ -369,12 +371,28 @@ static void pit_irq_timer_update(PITChannelState *s, 
int64_t current_time)
         return;
     expire_time = pit_get_next_transition_time(s, current_time);
     irq_level = pit_get_out1(s, current_time);
-    qemu_set_irq(s->irq, irq_level);
+    if(irq_level) {
+        if(!qemu_irq_raise(s->irq))
+            pit_irq_coalesced++;
+    } else {
+        qemu_irq_lower(s->irq);
+        if(pit_irq_coalesced > 0) {
+            if(qemu_irq_raise(s->irq))
+                pit_irq_coalesced--;
+            qemu_irq_lower(s->irq);
+        }
+    }
+
 #ifdef DEBUG_PIT
     printf("irq_level=%d next_delay=%f\n",
            irq_level,
            (double)(expire_time - current_time) / ticks_per_sec);
 #endif
+    if(pit_irq_coalesced && expire_time != -1) {
+        uint32_t div = ((pit_irq_coalesced >> 10) & 0x7f) + 2;
+        expire_time -= ((expire_time - current_time) / div);
+    }
+
     s->next_transition_time = expire_time;
     if (expire_time != -1)
         qemu_mod_timer(s->irq_timer, expire_time);