cpus: update comments

The returned value of cpu_get_clock() is plused with the offset,
so it is the time elapsed in virtual machine when vm is active.

Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc  Peter Crosthwaite <crosthwaite.peter@gmail.com>
Cc: Richard Henderson <rth@twiddle.net>
Signed-off-by: NCao jin <caoj.fnst@cn.fujitsu.com>
Message-Id: <1469790338-28990-4-git-send-email-caoj.fnst@cn.fujitsu.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

cpus.c

@@ -191,8 +191,12 @@ int64_t cpu_icount_to_ns(int64_t icount)
     return icount << icount_time_shift;
 }
 
-/* return the host CPU cycle counter and handle stop/restart */
-/* Caller must hold the BQL */
+/* return the time elapsed in VM between vm_start and vm_stop.  Unless
+ * icount is active, cpu_get_ticks() uses units of the host CPU cycle
+ * counter.
+ *
+ * Caller must hold the BQL
+ */
 int64_t cpu_get_ticks(void)
 {
     int64_t ticks;
@@ -229,7 +233,8 @@ static int64_t cpu_get_clock_locked(void)
     return time;
 }
 
-/* return the host CPU monotonic timer and handle stop/restart */
+/* Return the monotonic time elapsed in VM, i.e.,
+ * the time between vm_start and vm_stop */
 int64_t cpu_get_clock(void)
 {
     int64_t ti;