[PATCH v1] rtc/mc146818rtc: improve rtc performance

[Evanzhang <Evanzhang@archeros.com>]

under heavy workloads,irq_coalesced could up to 30+,
after modification EXTERNAL_INTERRUPT reduce by 40%

before:
Analyze events for all VMs, all VCPUs:
             VM-EXIT    Samples  Samples%     Time%

       EPT_VIOLATION    9833984    63.41%    15.96%
      IO_INSTRUCTION    2160843    13.93%    50.07%
  EXTERNAL_INTERRUPT    1949267    12.57%     0.89%
          APIC_WRITE     767795     4.95%     0.55%
         EOI_INDUCED     615308     3.97%     0.30%
                 HLT     130821     0.84%    31.77%

after:
Analyze events for all VMs, all VCPUs:
             VM-EXIT    Samples  Samples%     Time%

       EPT_VIOLATION    5238031    50.91%     6.44%
      IO_INSTRUCTION    2257658    21.94%    54.88%
  EXTERNAL_INTERRUPT    1160086    11.28%     0.61%
          APIC_WRITE     780454     7.59%     0.54%
         EOI_INDUCED     615309     5.98%     0.28%
                 HLT     179703     1.75%    36.87%

Signed-off-by: Evanzhang <Evanzhang@archeros.com>
---
 hw/rtc/mc146818rtc.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/hw/rtc/mc146818rtc.c b/hw/rtc/mc146818rtc.c
index c27c362..2995078 100644
--- a/hw/rtc/mc146818rtc.c
+++ b/hw/rtc/mc146818rtc.c
@@ -96,8 +96,8 @@ static void rtc_coalesced_timer_update(MC146818RtcState *s)
     if (s->irq_coalesced == 0) {
         timer_del(s->coalesced_timer);
     } else {
-        /* divide each RTC interval to 2 - 8 smaller intervals */
-        int c = MIN(s->irq_coalesced, 7) + 1;
+        /* divide each RTC interval to 2 - 32 smaller intervals */
+        int c = MIN(s->irq_coalesced, 31) + 1;
         int64_t next_clock = qemu_clock_get_ns(rtc_clock) +
             periodic_clock_to_ns(s->period / c);
         timer_mod(s->coalesced_timer, next_clock);
-- 
2.9.5