only call alarm_timer_update() on REG_B writes when relevant bits change

From: George Dunlap <george.dunlap@xxxxxxxxxxxxx>


---
 xen/arch/x86/hvm/rtc.c |    3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/xen/arch/x86/hvm/rtc.c b/xen/arch/x86/hvm/rtc.c
index a122300..d87a2f8 100644
--- a/xen/arch/x86/hvm/rtc.c
+++ b/xen/arch/x86/hvm/rtc.c
@@ -457,7 +457,8 @@ static int rtc_ioport_write(void *opaque, uint32_t addr, uint32_t data)
         s->hw.cmos_data[RTC_REG_B] = data;
         if ( (data ^ orig) & RTC_SET )
             check_update_timer(s);
-        alarm_timer_update(s);
+        if ( (data ^ orig) & (RTC_24H | RTC_DM_BINARY | RTC_SET) )
+            alarm_timer_update(s);
         break;
     case RTC_REG_C:
     case RTC_REG_D: