Re: [PATCH] hw/ptimer: Assert next_event is newer than last_event

[Peter Maydell]

On Sat, 21 Sep 2019 at 11:17, Philippe Mathieu-Daudé <address@hidden> wrote:
>
> If the period is too big, the 'delta * period' product result
> might overflow, resulting in a negative number, then the
> next_event ends before the last_event. This is buggy, as there
> is no forward progress. Assert this can not happen.
>
> Signed-off-by: Philippe Mathieu-Daudé <address@hidden>
> ---
>  hw/core/ptimer.c | 3 +++
>  1 file changed, 3 insertions(+)
>
> diff --git a/hw/core/ptimer.c b/hw/core/ptimer.c
> index d58e2dfdb0..88085d4c81 100644
> --- a/hw/core/ptimer.c
> +++ b/hw/core/ptimer.c
> @@ -125,6 +125,9 @@ static void ptimer_reload(ptimer_state *s, int 
> delta_adjust)
>
>      s->last_event = s->next_event;
>      s->next_event = s->last_event + delta * period;
> +    /* Verify forward progress */
> +    g_assert(s->next_event > s->last_event);
> +
>      if (period_frac) {
>          s->next_event += ((int64_t)period_frac * delta) >> 32;
>      }
> --

Can this only happen if a QEMU timer model using the ptimer
code has a bug, or is it guest-triggerable for some of our
timer models?

thanks
-- PMM