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

[David Vrabel <david.vrabel@xxxxxxxxxx>]

On 29/05/13 08:39, Jan Beulich wrote:
>>>> 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)?

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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