Re: [Xen-devel] [PATCH 3/4] x86/hvm: Avoid needlessly resetting the periodic timer.

[Tim Deegan <tim@xxxxxxx>]

At 13:41 +0000 on 28 Mar (1364478118), Jan Beulich wrote:
> >>> On 28.03.13 at 14:22, Tim Deegan <tim@xxxxxxx> wrote:
> > +            if ( !s->pt_active || (period != s->period) )
> > +            {
> > +                s->period = period;
> > +                s->pt_active = 1;
> 
> I think that by storing period_code rather than period, you could
> easily fold the two new fields "period" and "pt_active" into one
> (because period_code is zero if and only if the timer is inactive).

Good idea.  Here's v2:

commit bf6591a94ea3f77e40cad5662ab320e37de080c1
Author: Tim Deegan <tim@xxxxxxx>
Date:   Thu Mar 28 12:19:32 2013 +0000

    x86/hvm: Avoid needlessly resetting the periodic timer.
    
    Signed-off-by: Tim Deegan <tim@xxxxxxx>

diff --git a/xen/arch/x86/hvm/rtc.c b/xen/arch/x86/hvm/rtc.c
index 710ae89..49629b2 100644
--- a/xen/arch/x86/hvm/rtc.c
+++ b/xen/arch/x86/hvm/rtc.c
@@ -78,7 +78,10 @@ void rtc_periodic_interrupt(void *opaque)
 
     spin_lock(&s->lock);
     if ( s->hw.cmos_data[RTC_REG_C] & RTC_PF )
+    {
         destroy_periodic_time(&s->pt);
+        s->pt_code = 0;
+    }
     else
     {
         s->hw.cmos_data[RTC_REG_C] |= RTC_PF;
@@ -107,15 +110,21 @@ static void rtc_timer_update(RTCState *s)
     case RTC_REF_CLCK_4MHZ:
         if ( period_code != 0 )
         {
-            period = 1 << (period_code - 1); /* period in 32 Khz cycles */
-            period = DIV_ROUND(period * 1000000000ULL, 32768); /* in ns */
-            delta = period - ((NOW() - s->start_time) % period);
-            create_periodic_time(v, &s->pt, delta, period, RTC_IRQ, NULL, s);
+            if ( period_code != s->pt_code )
+            {
+                s->pt_code = period_code;
+                period = 1 << (period_code - 1); /* period in 32 Khz cycles */
+                period = DIV_ROUND(period * 1000000000ULL, 32768); /* in ns */
+                delta = period - ((NOW() - s->start_time) % period);
+                create_periodic_time(v, &s->pt, delta, period,
+                                     RTC_IRQ, NULL, s);
+            }
             break;
         }
         /* fall through */
     default:
         destroy_periodic_time(&s->pt);
+        s->pt_code = 0;
         break;
     }
 }
@@ -717,6 +726,7 @@ void rtc_reset(struct domain *d)
     RTCState *s = domain_vrtc(d);
 
     destroy_periodic_time(&s->pt);
+    s->pt_code = 0;
     s->pt.source = PTSRC_isa;
 }
 
diff --git a/xen/include/asm-x86/hvm/vpt.h b/xen/include/asm-x86/hvm/vpt.h
index 2e9c7d2..ea9df42 100644
--- a/xen/include/asm-x86/hvm/vpt.h
+++ b/xen/include/asm-x86/hvm/vpt.h
@@ -104,16 +104,16 @@ typedef struct RTCState {
     struct hvm_hw_rtc hw;
     /* RTC's idea of the current time */
     struct tm current_tm;
-    /* periodic timer */
-    s_time_t start_time;
-    /* second update */
-    struct periodic_time pt;
     /* update-ended timer */
     struct timer update_timer;
     struct timer update_timer2;
+    uint64_t next_update_time;
     /* alarm timer */
     struct timer alarm_timer;
-    uint64_t next_update_time;
+    /* periodic timer */
+    struct periodic_time pt;
+    s_time_t start_time;
+    int pt_code;
     uint32_t use_timer;
     spinlock_t lock;
 } RTCState;

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