Re: [Xen-devel] [PATCH 5/5] x86/xen: sync the CMOS RTC as well as the Xen wallclock

[David Vrabel <david.vrabel@xxxxxxxxxx>]

On 28/06/13 16:38, Thomas Gleixner wrote:
> On Thu, 27 Jun 2013, David Vrabel wrote:
> 
>> From: David Vrabel <david.vrabel@xxxxxxxxxx>
>>
>> Adjustments to Xen's persistent clock via update_persistent_clock()
>> don't actually persist, as the Xen wallclock is a software only clock
>> and modifications to it do not modify the underlying CMOS RTC.
>>
>> The x86_platform.set_wallclock hook can be used to keep the hardware
>> RTC synchronized (as on bare metal).  Because the Xen wallclock is now
>> kept synchronized by pvclock_gtod notifier, xen_set_wallclock() need
>> not do this and dom0 can simply use the native implementation.
> 
> I can understand that part, but ...
>  
>>  static int xen_pvclock_gtod_notify(struct notifier_block *nb, unsigned long 
>> was_set,
>>                                 void *priv)
>>  {
>> +    static struct timespec next;
>>      struct timespec now;
>>      struct xen_platform_op op;
>>  
>> -    if (!was_set)
>> -            return NOTIFY_OK;
>> -
>>      now = __current_kernel_time();
>>  
>> +    if (!was_set && timespec_compare(&now, &next) < 0)
>> +            return NOTIFY_OK;
>> +
>>      op.cmd = XENPF_settime;
>>      op.u.settime.secs = now.tv_sec;
>>      op.u.settime.nsecs = now.tv_nsec;
>>      op.u.settime.system_time = xen_clocksource_read();
>>  
>>      (void)HYPERVISOR_dom0_op(&op);
>> +
>> +    /*
>> +     * Don't update the wallclock for another 11 minutes. This is
>> +     * the same period as the sync_cmos_clock() work.
>> +     */
>> +    next = now;
>> +    next.tv_sec += 11*60;
>> +
> 
> How is this related to the changelog? /me is confused .....

Before:

Xen wallclock set when time is stepped.
Xen wallclock set every 11 minutes (by sync_cmos_clock()).
Hardware RTC never set.

After:

Xen wallclock set when time is stepped.
Xen wallclock set every 11 minutes (in pvclock gtod notifier).
Hardware RTC set every 11 minutes (by sync_cmos_clock()).

I'll update the changelog to be more descriptive:

  Adjustments to Xen's persistent clock via update_persistent_clock()
  don't actually persist, as the Xen wallclock is a software only clock
  and modifications to it do not modify the underlying CMOS RTC.

  The x86_platform.set_wallclock hook can be used to keep the hardware
  RTC synchronized (as on bare metal).  If (in dom0) we make the Xen
  wallclock periodically synchronized by the pvclock_gtod notifier, the
  set_wallclock hook need not update the Xen wallclock and the native
  implementation can be used.

Is that better?

David

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