[Xen-devel] [PATCH RFC v2 1/3] x86/hvm/rtc: Don't run the vpt timer when !REG_B.PIE.

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

From: Tim Deegan <tim@xxxxxxx>

If the guest has not asked for interrupts, don't run the vpt timer
to generate them.  This is a prerequisite for a patch to simplify how
the vpt interacts with the RTC, and also gets rid of a timer series in
Xen in a case where it's unlikely to be needed.

Instead, calculate the correct value for REG_C.PF whenever REG_C is
read or PIE is enabled.  This allow a guest to poll for the PF bit
while not asking for actual timer interrupts.  Such a guest would no
longer get the benefit of the vpt's timer modes.

Signed-off-by: Tim Deegan <tim@xxxxxxx>
Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>

---

Changes in v2:
 * Reduce scope of `now` in rtc_timer_update()
 * Merge PIE logic in REG_B write
 * Tightly couple setting s->period with creating/destroying timers, so the
   timer change gets properly recreated when the guest sets REG_B.PIE
---
 xen/arch/x86/hvm/rtc.c        |   58 ++++++++++++++++++++++++++++++++---------
 xen/include/asm-x86/hvm/vpt.h |    3 ++-
 2 files changed, 48 insertions(+), 13 deletions(-)

diff --git a/xen/arch/x86/hvm/rtc.c b/xen/arch/x86/hvm/rtc.c
index cdedefe..1455bc6 100644
--- a/xen/arch/x86/hvm/rtc.c
+++ b/xen/arch/x86/hvm/rtc.c
@@ -94,7 +94,7 @@ bool_t rtc_periodic_interrupt(void *opaque)
     {
         /* VM is ignoring its RTC; no point in running the timer */
         destroy_periodic_time(&s->pt);
-        s->pt_code = 0;
+        s->period = 0;
     }
     if ( !(s->hw.cmos_data[RTC_REG_C] & RTC_IRQF) )
         ret = 0;
@@ -103,6 +103,24 @@ bool_t rtc_periodic_interrupt(void *opaque)
     return ret;
 }
 
+/* Check whether the REG_C.PF bit should have been set by a tick since
+ * the last time we looked. This is used to track ticks when REG_B.PIE
+ * is clear; when PIE is set, PF ticks are handled by the VPT callbacks.  */
+static void check_for_pf_ticks(RTCState *s)
+{
+    s_time_t now;
+
+    if ( s->period == 0 || (s->hw.cmos_data[RTC_REG_B] & RTC_PIE) )
+        return;
+
+    now = NOW();
+    if ( (now - s->start_time) / s->period
+         != (s->check_ticks_since - s->start_time) / s->period )
+        s->hw.cmos_data[RTC_REG_C] |= RTC_PF;
+
+    s->check_ticks_since = now;
+}
+
 /* Enable/configure/disable the periodic timer based on the RTC_PIE and
  * RTC_RATE_SELECT settings */
 static void rtc_timer_update(RTCState *s)
@@ -125,24 +143,31 @@ static void rtc_timer_update(RTCState *s)
     case RTC_REF_CLCK_4MHZ:
         if ( period_code != 0 )
         {
-            if ( period_code != s->pt_code )
+            period = 1 << (period_code - 1); /* period in 32 Khz cycles */
+            period = DIV_ROUND(period * 1000000000ULL, 32768); /* in ns */
+            if ( period != s->period )
             {
-                s->pt_code = period_code;
-                period = 1 << (period_code - 1); /* period in 32 Khz cycles */
-                period = DIV_ROUND(period * 1000000000ULL, 32768); /* in ns */
+                s_time_t now = NOW();
+
                 if ( v->domain->arch.hvm_domain.params[HVM_PARAM_VPT_ALIGN] )
                     delta = 0;
                 else
-                    delta = period - ((NOW() - s->start_time) % period);
-                create_periodic_time(v, &s->pt, delta, period,
-                                     RTC_IRQ, NULL, s);
+                    delta = period - ((now - s->start_time) % period);
+                if ( s->hw.cmos_data[RTC_REG_B] & RTC_PIE )
+                {
+                    create_periodic_time(v, &s->pt, delta, period,
+                                         RTC_IRQ, NULL, s);
+                    s->period = period;
+                }
+                else
+                    s->check_ticks_since = now;
             }
             break;
         }
         /* fall through */
     default:
         destroy_periodic_time(&s->pt);
-        s->pt_code = 0;
+        s->period = 0;
         break;
     }
 }
@@ -484,14 +509,22 @@ static int rtc_ioport_write(void *opaque, uint32_t addr, 
uint32_t data)
             if ( orig & RTC_SET )
                 rtc_set_time(s);
         }
+        check_for_pf_ticks(s);
         s->hw.cmos_data[RTC_REG_B] = data;
         /*
          * If the interrupt is already set when the interrupt becomes
          * enabled, raise an interrupt immediately.
          */
         rtc_update_irq(s);
-        if ( (data & RTC_PIE) && !(orig & RTC_PIE) )
+        if ( (data ^ orig) & RTC_PIE )
+        {
+            if ( !(data & RTC_PIE) )
+            {
+                destroy_periodic_time(&s->pt);
+                s->period = 0;
+            }
             rtc_timer_update(s);
+        }
         if ( (data ^ orig) & RTC_SET )
             check_update_timer(s);
         if ( (data ^ orig) & (RTC_24H | RTC_DM_BINARY | RTC_SET) )
@@ -645,6 +678,7 @@ static uint32_t rtc_ioport_read(RTCState *s, uint32_t addr)
             ret |= RTC_UIP;
         break;
     case RTC_REG_C:
+        check_for_pf_ticks(s);
         ret = s->hw.cmos_data[s->hw.cmos_index];
         s->hw.cmos_data[RTC_REG_C] = 0x00;
         if ( (ret & RTC_IRQF) && !rtc_mode_is(s, no_ack) )
@@ -652,7 +686,7 @@ static uint32_t rtc_ioport_read(RTCState *s, uint32_t addr)
         rtc_update_irq(s);
         check_update_timer(s);
         alarm_timer_update(s);
-        rtc_timer_update(s);
+        s->pt_dead_ticks = 0;
         break;
     default:
         ret = s->hw.cmos_data[s->hw.cmos_index];
@@ -748,7 +782,7 @@ void rtc_reset(struct domain *d)
     RTCState *s = domain_vrtc(d);
 
     destroy_periodic_time(&s->pt);
-    s->pt_code = 0;
+    s->period = 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 87c3a66..9f48635 100644
--- a/xen/include/asm-x86/hvm/vpt.h
+++ b/xen/include/asm-x86/hvm/vpt.h
@@ -113,7 +113,8 @@ typedef struct RTCState {
     /* periodic timer */
     struct periodic_time pt;
     s_time_t start_time;
-    int pt_code;
+    s_time_t check_ticks_since;
+    int period;
     uint8_t pt_dead_ticks;
     uint32_t use_timer;
     spinlock_t lock;
-- 
1.7.10.4


_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxx
http://lists.xen.org/xen-devel