Re: [Xen-devel] [PATCH 2/2] x86/xen: sync the wallclock when the system time changes

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 28.05.13 at 20:22, David Vrabel <david.vrabel@xxxxxxxxxx> wrote:
> +static int xen_pvclock_gtod_notify(struct notifier_block *nb, unsigned long 
> unused,
> +                                void *priv)
> +{
> +     static struct timespec last, next;
> +     struct timespec now;
> +     struct timekeeper *tk = priv;
> +     struct xen_platform_op op;
> +     int ret;
> +
> +     /*
> +      * Set the Xen wallclock from Linux system time.
> +      *
> +      * dom0 hasn't historically maintained a very accurate
> +      * wallclock so guests don't expect it. We can therefore
> +      * reduce the number of expensive hypercalls by only updating
> +      * the wallclock every 0.5 s.
> +      */
> +
> +     now.tv_sec = tk->xtime_sec;
> +     now.tv_nsec = tk->xtime_nsec >> tk->shift;
> +
> +     if (timespec_compare(&now, &last) > 0
> +         && timespec_compare(&now, &next) < 0)

Is this really working the first time through (when both last and
next are still all zeros)?

Jan

> +             return 0;
> +
>       op.cmd = XENPF_settime;
> -     op.u.settime.secs = now->tv_sec;
> -     op.u.settime.nsecs = now->tv_nsec;
> +     op.u.settime.secs = now.tv_sec;
> +     op.u.settime.nsecs = now.tv_nsec;
>       op.u.settime.system_time = xen_clocksource_read();
>  
>       ret = HYPERVISOR_dom0_op(&op);
>       if (ret)
> -             return ret;
> +             return 0;
>  
> -     /* Set the hardware RTC. */
> -     return mach_set_rtc_mmss(now);
> +     last = now;
> +     next = timespec_add(now, ns_to_timespec(NSEC_PER_SEC / 2));
>  
> +     return 0;
>  }



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