[Qemu-devel] [PATCH v2 5/5] hpet 'driftfix': add code in hpet_timer() to compensate delayed callbacks and coalesced interrupts

[Ulrich Obergfell <uobergfe@redhat.com>]

Loss of periodic timer interrupts caused by delayed callbacks and by
interrupt coalescing is compensated by gradually injecting additional
interrupts during subsequent timer intervals, starting at a rate of
one additional interrupt per interval. If further interrupts are lost
while compensation is in progress, the rate is increased. A limit is
imposed on the rate and on the 'backlog' of lost interrupts that are
to be injected. If a guest o/s modifies the comparator register value
while compensation is in progress, the 'backlog' of lost interrupts
that are to be injected is scaled to the new value.

Injecting additional timer interrupts to compensate lost interrupts
can alleviate long term time drift. However, on a short time scale,
this method can have the side effect of making virtual machine time
intermittently pass faster than real time (depending on the guest's
time keeping algorithm). Compensation is disabled by default and can
be enabled for guests where this behaviour is acceptable.

Signed-off-by: Ulrich Obergfell <uobergfe@redhat.com>
---
 hw/hpet.c |   58 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++-
 1 files changed, 57 insertions(+), 1 deletions(-)

diff --git a/hw/hpet.c b/hw/hpet.c
index b5625fb..5a25a96 100644
--- a/hw/hpet.c
+++ b/hw/hpet.c
@@ -42,6 +42,9 @@
 
 #define HPET_MSI_SUPPORT        0
 
+#define MAX_IRQS_TO_INJECT (uint32_t)5000
+#define MAX_IRQ_RATE       (uint32_t)10
+
 struct HPETState;
 typedef struct HPETTimer {  /* timers */
     uint8_t tn;             /*timer number*/
@@ -309,8 +312,11 @@ static const VMStateDescription vmstate_hpet = {
 static void hpet_timer(void *opaque)
 {
     HPETTimer *t = opaque;
+    HPETState *s = t->state;
     uint64_t diff;
 
+    int irq_delivered = 0;
+    uint32_t irq_count = 0;
     uint64_t period = t->period;
     uint64_t cur_tick = hpet_get_ticks(t->state);
 
@@ -318,13 +324,36 @@ static void hpet_timer(void *opaque)
         if (t->config & HPET_TN_32BIT) {
             while (hpet_time_after(cur_tick, t->cmp)) {
                 t->cmp = (uint32_t)(t->cmp + t->period);
+                irq_count++;
             }
         } else {
             while (hpet_time_after64(cur_tick, t->cmp)) {
                 t->cmp += period;
+                irq_count++;
             }
         }
         diff = hpet_calculate_diff(t, cur_tick);
+        if (s->driftfix) {
+            if (t->saved_period != t->period) {
+                uint64_t tmp = (t->irqs_to_inject * t->saved_period)
+                                                  + t->ticks_not_accounted;
+                t->irqs_to_inject = tmp / t->period;
+                t->ticks_not_accounted = tmp % t->period;
+                t->saved_period = t->period;
+            }
+            t->irqs_to_inject += irq_count;
+            t->irqs_to_inject = MIN(t->irqs_to_inject, MAX_IRQS_TO_INJECT);
+            if (t->irqs_to_inject > 1) {
+                if (irq_count > 1) {
+                    t->irq_rate++;
+                    t->irq_rate = MIN(t->irq_rate, MAX_IRQ_RATE);
+                }
+                if (irq_count || t->divisor == 0) {
+                    t->divisor = t->irq_rate;
+                }
+                diff /= t->divisor--;
+            }
+        }
         qemu_mod_timer(t->qemu_timer,
                        qemu_get_clock(vm_clock) + (int64_t)ticks_to_ns(diff));
     } else if (t->config & HPET_TN_32BIT && !timer_is_periodic(t)) {
@@ -335,7 +364,22 @@ static void hpet_timer(void *opaque)
             t->wrap_flag = 0;
         }
     }
-    update_irq(t, 1);
+    if (s->driftfix && timer_is_periodic(t) && period != 0) {
+        if (t->irqs_to_inject) {
+            irq_delivered = update_irq(t, 1);
+            if (irq_delivered) {
+                t->irq_rate = MIN(t->irq_rate, t->irqs_to_inject);
+                t->irqs_to_inject--;
+            } else {
+                if (irq_count) {
+                    t->irq_rate++;
+                    t->irq_rate = MIN(t->irq_rate, MAX_IRQ_RATE);
+                }
+            }
+        }
+    } else {
+        update_irq(t, 1);
+    }
 }
 
 static void hpet_set_timer(HPETTimer *t)
@@ -674,6 +718,12 @@ static void hpet_reset(DeviceState *d)
         timer->config |=  0x00000004ULL << 32;
         timer->period = 0ULL;
         timer->wrap_flag = 0;
+
+        timer->saved_period = 0;
+        timer->ticks_not_accounted = 0;
+        timer->irqs_to_inject = 0;
+        timer->irq_rate = 1;
+        timer->divisor = 1;
     }
 
     s->hpet_counter = 0ULL;
@@ -734,6 +784,12 @@ static int hpet_init(SysBusDevice *dev)
         timer->qemu_timer = qemu_new_timer(vm_clock, hpet_timer, timer);
         timer->tn = i;
         timer->state = s;
+
+        timer->saved_period = 0;
+        timer->ticks_not_accounted = 0;
+        timer->irqs_to_inject = 0;
+        timer->irq_rate = 1;
+        timer->divisor = 1;
     }
 
     /* 64-bit main counter; LegacyReplacementRoute. */
-- 
1.6.2.5