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

[Jan Kiszka <jan.kiszka@siemens.com>]

On 2011-04-08 17:20, Ulrich Obergfell wrote:
> 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--;
> +            }
> +        }

Did I miss some change in the plan? I thought we were heading for a
generic, reusable driftfix tool box (or periodic timer service)? Or is
this intentionally an intermediate step?

>          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;

We call reset after initializing devices, at least for cold-plugged ones.

Jan

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