Re: [Xen-devel] [PATCH v3 5/6] xen/arm: introduce xen_read_wallclock

[Arnd Bergmann <arnd@xxxxxxxx>]

On Wednesday 11 November 2015 16:51:35 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));
> +
> +       /* time since system boot */
> +       delta = ktime_get_ns();
> +       delta += now.tv_sec * (u64)NSEC_PER_SEC + now.tv_nsec;
> +
> +       *ts = ns_to_timespec64(delta);
> +}

Looks correct to me and better than the previous versions, but you are still
converting from timespec64 to nanoseconds and back. While I previously
recommended going all the way to nanoseconds here, I guess this you
can even avoid the ns_to_timespec64() if you stay within timespec64
domain and replace the last lines with

        ktime_get_ts64(&ts_monotonic);
        *ts = timespec64_add(now, ts_monotonic);

which avoids both the multiplication and division.

        Arnd

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