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 10:48:41 +0100
From:	David Vrabel <david.vrabel@...rix.com>
To:	Jan Beulich <JBeulich@...e.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 29/05/13 08:39, Jan Beulich wrote:
>>>> 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)?

Yes. The first time through we want to set the wallclock and this test
is always false when last == next.

David
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