Re: [PATCH v2 3/3] hw/timer: ibex_timer.c: Add support for writes to mtime

["Philippe Mathieu-Daudé via" <qemu-devel@nongnu.org>]

On 22/9/22 17:58, Tyler Ng wrote:
> 1. Adds fields to hold the value of mtime in timer_upper0 and timer_lower0.
> 
> 2. Changes the read and write functions to use the mtime fields.
> 
> 3. Updates the value of mtime in update_mtime() by extrapolating the
> time elapsed. This will need to change if/when the prescalar is
> implemented.
> 
> 4. Adds a qtest for the ibex timer.
> 
> Signed-off-by: Tyler Ng <tkng@rivosinc.com>
> ---
>   hw/timer/ibex_timer.c         |  98 +++++++++++++------
>   include/hw/timer/ibex_timer.h |   6 ++
>   tests/qtest/ibex-timer-test.c | 178 ++++++++++++++++++++++++++++++++++
>   tests/qtest/meson.build       |   3 +-
>   4 files changed, 256 insertions(+), 29 deletions(-)
>   create mode 100644 tests/qtest/ibex-timer-test.c

> -static void ibex_timer_update_irqs(IbexTimerState *s)
> +/*
> + * The goal of this function is to:
> + * 1. Check if the timer is enabled. If not, return false,
> + * 2. Calculate the amount of time that has passed since.
> + * 3. Extrapolate the number of ticks that have passed, and add it to `mtime`.
> + * 4. Return true.
> + */
> +static bool update_mtime(IbexTimerState *s)
>   {
> -    uint64_t value = s->timer_compare_lower0 |
> -                         ((uint64_t)s->timer_compare_upper0 << 32);
> -    uint64_t next, diff;
> -    uint64_t now = cpu_riscv_read_rtc(s->timebase_freq);
> -
>       if (!(s->timer_ctrl & R_CTRL_ACTIVE_MASK)) {
> -        /* Timer isn't active */
> +        return false;
> +    }
> +    /* Get the time then extrapolate the number of ticks that have elapsed */
> +    uint64_t mtime = get_mtime(s);
> +    int64_t now = qemu_clock_get_ns(QEMU_CLOCK_VIRTUAL);
> +    int64_t elapsed = now - s->timer_last_update;
> +    if (elapsed < 0) {
> +        /* We jumped back in time. */
> +        mtime -= muldiv64((uint64_t)(-elapsed), s->timebase_freq,
> +                           NANOSECONDS_PER_SECOND);
> +    } else {
> +        mtime += muldiv64(elapsed, s->timebase_freq, NANOSECONDS_PER_SECOND);
> +    }
> +    s->timer_lower0 = mtime & 0xffffffff;
> +    s->timer_upper0 = (mtime >> 32) & 0xffffffff;

Could use extract64(mtime, 0, 32) and extract64(mtime, 32, 32);

> +    /* update last-checkpoint timestamp */
> +    s->timer_last_update = now;
> +    return true;
> +}