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

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Date:	Wed, 29 May 2013 08:39:39 +0100
From:	"Jan Beulich" <JBeulich@...e.com>
To:	"David Vrabel" <david.vrabel@...rix.com>
Cc:	"John Stultz" <john.stultz@...aro.org>, <xen-devel@...ts.xen.org>,
	"Konrad Rzeszutek Wilk" <konrad.wilk@...cle.com>,
	<linux-kernel@...r.kernel.org>
Subject: Re: [Xen-devel] [PATCH 2/2] x86/xen: sync the wallclock when
 the system time changes

>>> On 28.05.13 at 20:22, David Vrabel <david.vrabel@...rix.com> 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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