Re: [RFC PATCH Mach] Add a mach host operation which returns elapsed time since bootup

[Diego Nieto Cid]

El mar, 17 dic 2024 a las 22:21, Zhaoming Luo (<zhmingluo@163.com>) escribió:
>
> The precision of this implmentation is 10ms. Not sure how to do with the
> possible data race.
>
> Signed-off-by: Zhaoming Luo <zhmingluo@163.com>
> ---
>  include/mach/mach_host.defs |  7 +++++++
>  kern/mach_clock.c           | 20 ++++++++++++++++++++
>  2 files changed, 27 insertions(+)
>
> diff --git a/include/mach/mach_host.defs b/include/mach/mach_host.defs
> index 8fd9d6b3..67f72cda 100644
> --- a/include/mach/mach_host.defs
> +++ b/include/mach/mach_host.defs
> @@ -386,3 +386,10 @@ routine    host_adjust_time64(
>  routine        host_get_kernel_version(
>                 host            : host_t;
>         out     kernel_version  : new_kernel_version_t);
> +
> +/*
> + *     Get the elapsed time on this host.
> + */
> +routine        host_get_elapsed_time(
> +               host            : host_t;
> +       out     elapsed_time    : time_value_t);

If I'm not mistaken, this is most commonly called `uptime` (like in
`host_get_uptime` or similar)

> +
> +       read_elapsed_ticks = elapsed_ticks;
> +       elapsed_usec = read_elapsed_ticks * tick;
> +       elapsed_time->seconds = elapsed_usec / MICROSECONDS_IN_ONE_SECOND;
> +       elapsed_time->microseconds = elapsed_usec % 
> MICROSECONDS_IN_ONE_SECOND;

`read_elapsed_ticks` is only used to calculate elapsed_usec and thus
may be inlined, removing the otherwise unused variable

As for the data races, I believe you can use atomic builtins[1], like below:

    elapsed_usec = __sync_fetch_and_add(&elapsed_tikcs, 0) * tick

Or you could also use the lock used to increment `elapsed_ticks`:

     s = simple_lock_irq(&timer_lock);
    elapsed_usec = elapsed_tikcs * tick;
    simple_unlock_irq(s, &timer_lock);

I'm not sure which works best.

Cheers

---
[1] https://gcc.gnu.org/onlinedocs/gcc-4.1.2/gcc/Atomic-Builtins.html