From: Ilya Leoshkevich <iii@linux.ibm.com>

Suppose TOD clock value is 0x1111111111111111 and clock-comparator
value is 0, in which case clock-comparator interruption should occur
immediately.

With the current code, tod2time(env->ckc - td->base.low) ends up being
a very large number, so this interruption never happens.

Fix by firing the timer immediately if env->ckc < td->base.low.

Cc: qemu-stable@nongnu.org
Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Message-ID: <20251016175954.41153-2-iii@linux.ibm.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>
(cherry picked from commit df7e9243d540ee130f044f975af8de33c45f5299)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>

diff --git a/target/s390x/tcg/misc_helper.c b/target/s390x/tcg/misc_helper.c
index 31266aeda4..06b75aa453 100644
--- a/target/s390x/tcg/misc_helper.c
+++ b/target/s390x/tcg/misc_helper.c
@@ -199,11 +199,15 @@ static void update_ckc_timer(CPUS390XState *env)
         return;
     }
 
-    /* difference between origins */
-    time = env->ckc - td->base.low;
+    if (env->ckc < td->base.low) {
+        time = 0;
+    } else {
+        /* difference between origins */
+        time = env->ckc - td->base.low;
 
-    /* nanoseconds */
-    time = tod2time(time);
+        /* nanoseconds */
+        time = tod2time(time);
+    }
 
     timer_mod(env->tod_timer, time);
 }
-- 
2.47.3