Re: [PATCH v4 1/4] target/s390x: Fix missing interrupts for small CKC values

[Thomas Huth <thuth@redhat.com>]

On 16/10/2025 19.58, Ilya Leoshkevich wrote:
> 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>
> ---

  Hi Ilya,

this patch unfortunately broke reverse debugging on the s390x target. 
Something like this used to work before:

  qemu-img create -f qcow2 /tmp/disk.qcow2 2G
  ./qemu-system-s390x -nographic \
    -icount shift=6,rr=record,rrfile=replay.bin,rrsnapshot=init \
    -net none -drive file=/tmp/disk.qcow2,if=none
  ./qemu-system-s390x -nographic \
    -icount shift=6,rr=replay,rrfile=replay.bin,rrsnapshot=init \
    -net none -drive file=/tmp/disk.qcow2,if=none

With this commit and later, the replay hangs somewhere in an endless loop.
Do you have any ideas what could go wrong here?

  Thanks,
   Thomas


> diff --git a/target/s390x/tcg/misc_helper.c b/target/s390x/tcg/misc_helper.c
> index 6d9d601d29a..215b5b9d933 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);
>   }