[PATCH v2 6/7] xen/arm: introduce xen_read_wallclock

Arnd Bergmann
2015-11-10T12:35:41Z

On Tuesday 10 November 2015 11:57:54 Stefano Stabellini wrote:
> +static void xen_read_wallclock(struct timespec64 *ts)
> +{
> + u32 version;
> + u64 delta;
> + struct timespec64 now;
> + struct shared_info *s = HYPERVISOR_shared_info;
> + struct pvclock_wall_clock *wall_clock = &(s->wc);
> +
> + /* get wallclock at system boot */
> + do {
> + version = wall_clock->version;
> + rmb(); /* fetch version before time */
> + now.tv_sec = ((uint64_t)wall_clock->sec_hi << 32) | wall_clock->sec;
> + now.tv_nsec = wall_clock->nsec;
> + rmb(); /* fetch time before checking version */
> + } while ((wall_clock->version & 1) || (version != wall_clock->version));
> +
> + delta = arch_timer_read_counter() * (u64)NSEC_PER_SEC;
> + do_div(delta, arch_timer_get_rate()); /* time since system boot */
> + delta += now.tv_sec * (u64)NSEC_PER_SEC + now.tv_nsec;
> +
> + now.tv_nsec = do_div(delta, NSEC_PER_SEC);
> + now.tv_sec = delta;
> +
> + set_normalized_timespec64(ts, now.tv_sec, now.tv_nsec);
> +
> +}

Instead of the two do_div(), I would do the entire calculation in
terms of nanoseconds and then call ns_to_timespec64() in the end
instead of set_normalized_timespec64(). That is just an optimization
though, your version looks correct as well.

Arnd