Re: [PATCH v1 2/2] target/arm: only set the nexttick timer if !ISTATUS

[Peter Maydell <peter.maydell@linaro.org>]

On Tue, 28 Jul 2020 at 15:10, Alex Bennée <alex.bennee@linaro.org> wrote:
>
> Otherwise we have an unfortunate interaction with -count sleep=off
> which means we fast forward time when we don't need to. The easiest
> way to trigger it was to attach to the gdbstub and place a break point
> at the timers IRQ routine. Once the timer fired setting the next event
> at INT_MAX then qemu_start_warp_timer would skip to the end.
>
> Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
> ---
>  target/arm/helper.c | 35 ++++++++++++++++++++++-------------
>  1 file changed, 22 insertions(+), 13 deletions(-)
>
> diff --git a/target/arm/helper.c b/target/arm/helper.c
> index c69a2baf1d3..ec1b84cf0fd 100644
> --- a/target/arm/helper.c
> +++ b/target/arm/helper.c
> @@ -2683,7 +2683,7 @@ static void gt_recalc_timer(ARMCPU *cpu, int timeridx)
>          uint64_t count = gt_get_countervalue(&cpu->env);
>          /* Note that this must be unsigned 64 bit arithmetic: */
>          int istatus = count - offset >= gt->cval;
> -        uint64_t nexttick;
> +        uint64_t nexttick = 0;
>          int irqstate;
>
>          gt->ctl = deposit32(gt->ctl, 2, 1, istatus);
> @@ -2692,21 +2692,30 @@ static void gt_recalc_timer(ARMCPU *cpu, int timeridx)
>          qemu_set_irq(cpu->gt_timer_outputs[timeridx], irqstate);
>
>          if (istatus) {
> -            /* Next transition is when count rolls back over to zero */
> -            nexttick = UINT64_MAX;
> +            /*
> +             * The IRQ status of the timer will persist until:
> +             *   - CVAL is changed or
> +             *   - ENABLE is changed
> +             *
> +             * There is no point re-arming the timer for some far
> +             * flung future - currently it just is.
> +             */
> +            timer_del(cpu->gt_timer[timeridx]);

Why do we delete the timer for this case of "next time we need to
know is massively in the future"...

>          } else {
>              /* Next transition is when we hit cval */
>              nexttick = gt->cval + offset;
> -        }
> -        /* Note that the desired next expiry time might be beyond the
> -         * signed-64-bit range of a QEMUTimer -- in this case we just
> -         * set the timer for as far in the future as possible. When the
> -         * timer expires we will reset the timer for any remaining period.
> -         */
> -        if (nexttick > INT64_MAX / gt_cntfrq_period_ns(cpu)) {
> -            timer_mod_ns(cpu->gt_timer[timeridx], INT64_MAX);
> -        } else {
> -            timer_mod(cpu->gt_timer[timeridx], nexttick);
> +
> +            /*
> +             * It is possible the next tick is beyond the
> +             * signed-64-bit range of a QEMUTimer but currently the
> +             * timer system doesn't support a run time of more the 292
> +             * odd years so we set it to INT_MAX in this case.
> +             */
> +            if (nexttick > INT64_MAX / gt_cntfrq_period_ns(cpu)) {
> +                timer_mod_ns(cpu->gt_timer[timeridx], INT64_MAX);

...but here we handle the similar case by "set a timeout for
INT64_MAX" ?

> +            } else {
> +                timer_mod(cpu->gt_timer[timeridx], nexttick);
> +            }
>          }
>          trace_arm_gt_recalc(timeridx, irqstate, nexttick);
>      } else {
> --

thanks
-- PMM