Xen devel - Jun 2013 - [PATCHv5 0/4] xen: maintain an accurate persistent clock in more cases

Xen guests use the Xen wallclock as their persistent clock.  This is a
software only clock in the hypervisor that is used by guests instead
of a real hardware RTC.

The kernel has limited support for updating the persistent clock or
RTC when NTP is synced.  This has the following limitations:

* The persistent clock is not updated on step changes.  This leaves a
  window where it will be incorrect (while NTP resyncs).

* Xen guests use the Xen wallclock as their persistent clock.  dom0
  maintains this clock so it is persistent for domUs but not dom0
  itself.

These limitations mean that guests started before NTP is synchronized
will start with an incorrect wallclock time and the hardware RTC will
not be updated (as on bare metal).

These series fixes the above limitations and depends on "x86: increase
precision of x86_platform.get/set_wallclock()" which was previously
posted.

Changes since v4:

Dropped the change to disable non-boot CPUs during suspend on Xen as
migration downtime was too poor.  Instead, provide
hrtimers_late_resume() for use by Xen''s resume code to replace the
call of clock_was_set().  Fix two unused variable warnings.

Changes since v3:

Add a new clock_was_set notifier chain. Use this instead of direct
calls to clock_was_set() from the timekeeping code.  Use this notifier
and a new timer to synchronize the Xen wallclock.

Changes since v2:

Don''t peek at the timekeeper internals (use __current_kernel_time()
instead).  Use the native set_wallclock hook in dom0.

Changes since v1:

Reworked to use the pvclock_gtod notifier to sync the wallclock (this
looked similar to what a KVM host does).  update_persistent_clock()
will now only update the CMOS RTC.

David

Xen guests use the Xen wallclock as their persistent clock.  This is a
software only clock in the hypervisor that is used by guests instead
of a real hardware RTC.

The kernel has limited support for updating the persistent clock or
RTC when NTP is synced.  This has the following limitations:

* The persistent clock is not updated on step changes.  This leaves a
  window where it will be incorrect (while NTP resyncs).

* Xen guests use the Xen wallclock as their persistent clock.  dom0
  maintains this clock so it is persistent for domUs but not dom0
  itself.

These limitations mean that guests started before NTP is synchronized
will start with an incorrect wallclock time and the hardware RTC will
not be updated (as on bare metal).

These series fixes the above limitations and depends on "x86: increase
precision of x86_platform.get/set_wallclock()" which was previously
posted.

Changes since v4:

Dropped the change to disable non-boot CPUs during suspend on Xen as
migration downtime was too poor.  Instead, provide
hrtimers_late_resume() for use by Xen''s resume code to replace the
call of clock_was_set().  Fix two unused variable warnings.

Changes since v3:

Add a new clock_was_set notifier chain. Use this instead of direct
calls to clock_was_set() from the timekeeping code.  Use this notifier
and a new timer to synchronize the Xen wallclock.

Changes since v2:

Don''t peek at the timekeeper internals (use __current_kernel_time()
instead).  Use the native set_wallclock hook in dom0.

Changes since v1:

Reworked to use the pvclock_gtod notifier to sync the wallclock (this
looked similar to what a KVM host does).  update_persistent_clock()
will now only update the CMOS RTC.

David

On 20/06/13 20:13, David Vrabel wrote:
> Xen guests use the Xen wallclock as their persistent clock.  This is a
> software only clock in the hypervisor that is used by guests instead
> of a real hardware RTC.
Sorry, I accidentally specified the wrong revision range and only
include 2 out 4 of the patches in the series.

I have resent the complete series.

David

On 06/20/2013 12:16 PM, David Vrabel wrote:
> Xen guests use the Xen wallclock as their persistent clock.  This is a
> software only clock in the hypervisor that is used by guests instead
> of a real hardware RTC.
>
> The kernel has limited support for updating the persistent clock or
> RTC when NTP is synced.  This has the following limitations:
>
> * The persistent clock is not updated on step changes.  This leaves a
>    window where it will be incorrect (while NTP resyncs).
>
> * Xen guests use the Xen wallclock as their persistent clock.  dom0
>    maintains this clock so it is persistent for domUs but not dom0
>    itself.
>
> These limitations mean that guests started before NTP is synchronized
> will start with an incorrect wallclock time and the hardware RTC will
> not be updated (as on bare metal).
>
> These series fixes the above limitations and depends on "x86: increase
> precision of x86_platform.get/set_wallclock()" which was previously
> posted.
>
> Changes since v4:
>
> Dropped the change to disable non-boot CPUs during suspend on Xen as
> migration downtime was too poor.  Instead, provide
> hrtimers_late_resume() for use by Xen''s resume code to replace the
> call of clock_was_set().  Fix two unused variable warnings.
Ok, I''ve got these 4 in my pending stack. As long as Thomas
doesn''t
object to the first two, and it doesn''t run into any trouble in
testing,
I''ll send them along for 3.12. (Acks from Xen maintainers would be nice
for the last two as well).

Thanks for all the effort through all the revisions here!

thanks
-john

On Thu, Jun 20, 2013 at 01:03:34PM -0700, John Stultz
wrote:
> On 06/20/2013 12:16 PM, David Vrabel wrote:
> >Xen guests use the Xen wallclock as their persistent clock.  This is a
> >software only clock in the hypervisor that is used by guests instead
> >of a real hardware RTC.
> >
> >The kernel has limited support for updating the persistent clock or
> >RTC when NTP is synced.  This has the following limitations:
> >
> >* The persistent clock is not updated on step changes.  This leaves a
> >   window where it will be incorrect (while NTP resyncs).
> >
> >* Xen guests use the Xen wallclock as their persistent clock.  dom0
> >   maintains this clock so it is persistent for domUs but not dom0
> >   itself.
> >
> >These limitations mean that guests started before NTP is synchronized
> >will start with an incorrect wallclock time and the hardware RTC will
> >not be updated (as on bare metal).
> >
> >These series fixes the above limitations and depends on "x86:
increase
> >precision of x86_platform.get/set_wallclock()" which was
previously
> >posted.
> >
> >Changes since v4:
> >
> >Dropped the change to disable non-boot CPUs during suspend on Xen as
> >migration downtime was too poor.  Instead, provide
> >hrtimers_late_resume() for use by Xen''s resume code to replace
the
> >call of clock_was_set().  Fix two unused variable warnings.
> 
> Ok, I''ve got these 4 in my pending stack. As long as Thomas
doesn''t
> object to the first two, and it doesn''t run into any trouble in
> testing, I''ll send them along for 3.12. (Acks from Xen maintainers
> would be nice for the last two as well).
Please consider them Acked-by.

Thanks!
> 
> Thanks for all the effort through all the revisions here!
> 
> thanks
> -john
>