On Thu, Apr 28, 2011 at 04:25:00PM +0200, 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. The injection of additional
> interrupts is based on a backlog of unaccounted HPET clock periods
> (new HPETTimer field 'ticks_not_accounted'). The backlog increases
> due to delayed callbacks and coalesced interrupts, and it decreases
> if an interrupt was injected successfully. If the backlog increases
> while compensation is still in progress, the rate at which additional
> interrupts are injected is increased too. A limit is imposed on the
> backlog and on the rate.
>
> 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 slower and 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 may be
> acceptable.
>
> Signed-off-by: Ulrich Obergfell <uobergfe@xxxxxxxxxx>
> ---
> hw/hpet.c | 63 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++-
> 1 files changed, 61 insertions(+), 2 deletions(-)
>
> diff --git a/hw/hpet.c b/hw/hpet.c
> index 35466ae..92d5f58 100644
> --- a/hw/hpet.c
> +++ b/hw/hpet.c
> @@ -32,6 +32,7 @@
> #include "sysbus.h"
> #include "mc146818rtc.h"
> #include "sysemu.h"
> +#include <assert.h>
>
> //#define HPET_DEBUG
> #ifdef HPET_DEBUG
> @@ -42,6 +43,9 @@
>
> #define HPET_MSI_SUPPORT 0
>
> +#define MAX_TICKS_NOT_ACCOUNTED (uint64_t)500000000 /* 5 sec */
> +#define MAX_IRQ_RATE (uint32_t)10
> +
> struct HPETState;
> typedef struct HPETTimer { /* timers */
> uint8_t tn; /*timer number*/
> @@ -326,28 +330,63 @@ static const VMStateDescription vmstate_hpet = {
> }
> };
>
> +static bool hpet_timer_has_tick_backlog(HPETTimer *t)
> +{
> + uint64_t backlog = t->ticks_not_accounted - (t->period + t->prev_period);
> + return (backlog >= t->period);
> +}
> +
> /*
> * timer expiration callback
> */
> 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);
>
> + if (s->driftfix && !t->ticks_not_accounted) {
> + t->ticks_not_accounted = t->prev_period = t->period;
> + }
> if (timer_is_periodic(t) && period != 0) {
> if (t->config & HPET_TN_32BIT) {
> while (hpet_time_after(cur_tick, t->cmp)) {
> t->cmp = (uint32_t)(t->cmp + t->period);
> + t->ticks_not_accounted += t->period;
> + irq_count++;
> }
> } else {
> while (hpet_time_after64(cur_tick, t->cmp)) {
> t->cmp += period;
> + t->ticks_not_accounted += period;
> + irq_count++;
> }
> }
> diff = hpet_calculate_diff(t, cur_tick);
> + if (s->driftfix) {
> + if (t->ticks_not_accounted > MAX_TICKS_NOT_ACCOUNTED) {
> + t->ticks_not_accounted = t->period + t->prev_period;
> + }
> + if (hpet_timer_has_tick_backlog(t)) {
> + if (t->irq_rate == 1 || irq_count > 1) {
> + t->irq_rate++;
> + t->irq_rate = MIN(t->irq_rate, MAX_IRQ_RATE);
> + }
> + if (t->divisor == 0) {
> + assert(irq_count);
> + }
> + if (irq_count) {
> + t->divisor = t->irq_rate;
> + }
> + diff /= t->divisor--;
> + } else {
> + t->irq_rate = 1;
> + }
> + }
> qemu_mod_timer(t->qemu_timer,
> qemu_get_clock_ns(vm_clock) + (int64_t)ticks_to_ns(diff));
> } else if (t->config & HPET_TN_32BIT && !timer_is_periodic(t)) {
> @@ -358,7 +397,22 @@ 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->ticks_not_accounted >= t->period + t->prev_period) {
> + irq_delivered = update_irq(t, 1);
> + if (irq_delivered) {
> + t->ticks_not_accounted -= t->prev_period;
> + t->prev_period = t->period;
> + } else {
> + if (irq_count) {
> + t->irq_rate++;
> + t->irq_rate = MIN(t->irq_rate, MAX_IRQ_RATE);
> + }
> + }
> + }
> + } else {
> + update_irq(t, 1);
> + }
> }
Hi Ulrich,
Whats prev_period for, since in practice the period will not change
between interrupts (OS programs comparator once, or perhaps twice during
bootup) ?
Other than that, shouldnt reset accounting variables to init state on
write to GLOBAL_ENABLE_CFG / writes to main counter?
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