[PATCH v3 1/2] x86: restore pv_rtc_handler() invocation

[Jan Beulich <jbeulich@xxxxxxxx>]

This was lost when making the logic accessible to PVH Dom0.

While doing so make the access to the global function pointer safe
against races (as noticed by Roger): The only current user wants to be
invoked just once (but can tolerate to be invoked multiple times),
zapping the pointer at that point.

Fixes: 835d8d69d96a ("x86/rtc: provide mediated access to RTC for PVH dom0")
Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
---
v3: Latch pointer under lock.
v2: New.

--- a/xen/arch/x86/time.c
+++ b/xen/arch/x86/time.c
@@ -1148,6 +1148,8 @@ void rtc_guest_write(unsigned int port,
 
     switch ( port )
     {
+        typeof(pv_rtc_handler) hook;
+
     case RTC_PORT(0):
         /*
          * All PV domains (and PVH dom0) are allowed to write to the latched
@@ -1160,6 +1162,14 @@ void rtc_guest_write(unsigned int port,
     case RTC_PORT(1):
         if ( !ioports_access_permitted(currd, RTC_PORT(0), RTC_PORT(1)) )
             break;
+
+        spin_lock_irqsave(&rtc_lock, flags);
+        hook = pv_rtc_handler;
+        spin_unlock_irqrestore(&rtc_lock, flags);
+
+        if ( hook )
+            hook(currd->arch.cmos_idx & 0x7f, data);
+
         spin_lock_irqsave(&rtc_lock, flags);
         outb(currd->arch.cmos_idx & 0x7f, RTC_PORT(0));
         outb(data, RTC_PORT(1));