migration: Do not re-read the clock on pre_save in case of paused guest

Re-read the timebase before migrate was ported from x86 commit:
   6053a86f: kvmclock: reduce kvmclock difference on migration

The clock move makes the guest knows about the paused time between
the stop and migrate commands. This is an issue in an already-paused
VM because some side effects, like process stalls, could happen
after migration.

So, this patch checks the runstate of guest in the pre_save handler and
do not re-reads the timebase in case of paused state (cold migration).

Signed-off-by: Maxiwell S. Garcia <maxiwell@linux.ibm.com>
Message-Id: <20190711194702.26598-1-maxiwell@linux.ibm.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

hw/ppc/ppc.c

@@ -1011,6 +1011,8 @@ static void timebase_save(PPCTimebase *tb)
      * there is no need to update it from KVM here
      */
     tb->guest_timebase = ticks + first_ppc_cpu->env.tb_env->tb_offset;
+
+    tb->runstate_paused = runstate_check(RUN_STATE_PAUSED);
 }
 
 static void timebase_load(PPCTimebase *tb)
@@ -1054,9 +1056,9 @@ void cpu_ppc_clock_vm_state_change(void *opaque, int running,
 }
 
 /*
- * When migrating, read the clock just before migration,
- * so that the guest clock counts during the events
- * between:
+ * When migrating a running guest, read the clock just
+ * before migration, so that the guest clock counts
+ * during the events between:
  *
  *  * vm_stop()
  *  *
@@ -1071,7 +1073,10 @@ static int timebase_pre_save(void *opaque)
 {
     PPCTimebase *tb = opaque;
 
-    timebase_save(tb);
+    /* guest_timebase won't be overridden in case of paused guest */
+    if (!tb->runstate_paused) {
+        timebase_save(tb);
+    }
 
     return 0;
 }

target/ppc/cpu-qom.h

@@ -201,6 +201,7 @@ typedef struct PowerPCCPUClass {
 typedef struct PPCTimebase {
     uint64_t guest_timebase;
     int64_t time_of_the_day_ns;
+    bool runstate_paused;
 } PPCTimebase;
 
 extern const VMStateDescription vmstate_ppc_timebase;