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

[Gleb Natapov <gleb@redhat.com>]

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 <gleb@redhat.com>
---

 hw/i8254.c |   23 ++++++++++++++++++++++-
 1 files changed, 22 insertions(+), 1 deletions(-)

diff --git a/hw/i8254.c b/hw/i8254.c
index 4813b03..1a43ffc 100644
--- a/hw/i8254.c
+++ b/hw/i8254.c
@@ -49,6 +49,7 @@ typedef struct PITChannelState {
     int64_t count_load_time;
     /* irq handling */
     int64_t next_transition_time;
+    uint32_t irq_coalesced;
     QEMUTimer *irq_timer;
     qemu_irq irq;
 } PITChannelState;
@@ -228,6 +229,9 @@ static inline void pit_load_count(PITChannelState *s, int val)
 {
     if (val == 0)
         val = 0x10000;
+    /* recalculate how much IRQs to inject based on new timer value */
+    if(s->irq_coalesced)
+        s->irq_coalesced = (s->irq_coalesced * s->count) / val;
     s->count_load_time = qemu_get_clock(vm_clock);
     s->count = val;
     pit_irq_timer_update(s, s->count_load_time);
@@ -369,12 +373,28 @@ 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))
+            s->irq_coalesced++;
+    } else {
+        qemu_irq_lower(s->irq);
+        if(s->irq_coalesced > 0) {
+            if(qemu_irq_raise(s->irq))
+                s->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(s->irq_coalesced && expire_time != -1) {
+        uint32_t div = ((s->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);
@@ -459,6 +479,7 @@ static void pit_reset(void *opaque)
         s = &pit->channels[i];
         s->mode = 3;
         s->gate = (i != 2);
+        s->irq_coalesced = 0;
         pit_load_count(s, 0);
     }
 }