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

[Dor Laor <dlaor@redhat.com>]

Gleb Natapov wrote:
> 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
> RTC is used as a time source.
>
> Signed-off-by: Gleb Natapov <gleb@redhat.com>
> ---
>
>  hw/mc146818rtc.c |   11 +++++++++--
>  1 files changed, 9 insertions(+), 2 deletions(-)
>
> diff --git a/hw/mc146818rtc.c b/hw/mc146818rtc.c
> index 30bb044..505e117 100644
> --- a/hw/mc146818rtc.c
> +++ b/hw/mc146818rtc.c
> @@ -66,6 +66,7 @@ struct RTCState {
>      int64_t next_periodic_time;
>      /* second update */
>      int64_t next_second_time;
> +    uint32_t irq_coalesced;
>      QEMUTimer *second_timer;
>      QEMUTimer *second_timer2;
>  };
> @@ -101,7 +102,8 @@ static void rtc_periodic_timer(void *opaque)
>  
>      rtc_timer_update(s, s->next_periodic_time);
>      s->cmos_data[RTC_REG_C] |= 0xc0;
> -    qemu_irq_raise(s->irq);
> +    if(!qemu_irq_raise(s->irq))
> +        s->irq_coalesced++;
>  }
>  
>  static void cmos_ioport_write(void *opaque, uint32_t addr, uint32_t data)
> @@ -360,7 +362,12 @@ static uint32_t cmos_ioport_read(void *opaque, uint32_t addr)
>          case RTC_REG_C:
>              ret = s->cmos_data[s->cmos_index];
>              qemu_irq_lower(s->irq);
> -            s->cmos_data[RTC_REG_C] = 0x00;
> +            if(!s->irq_coalesced) {
> +                s->cmos_data[RTC_REG_C] = 0x00;
> +            } else {
> +                if(qemu_irq_raise(s->irq))
> +                    s->irq_coalesced--;
> +            }
>              break;
>          default:
>              ret = s->cmos_data[s->cmos_index];
>
>
>   
The rtc needs to recalculate the irq_coalesced counter if the frequency 
was changed, similarly to the pit patch.