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SubjectRe: [PATCH 3/3] x86/xen: sync the CMOS RTC as well as the Xen wallclock
On 14/05/13 01:52, John Stultz wrote:
> On 05/13/2013 10:56 AM, David Vrabel wrote:
>> From: David Vrabel <david.vrabel@citrix.com>
>>
>> If NTP is used in dom0 and it is synchronized to its clock source,
>> then the kernel will periodically synchronize the Xen wallclock with
>> the system time. Updates to the Xen wallclock do not persist across
>> reboots, so also synchronize the CMOS RTC (as on bare metal).
>
> Sorry again, not getting this one either.
>
> So normally in this case we're using the Xen wallclock as the underlying
> source for the persistent_clock here, my understanding is we use this
> instead of the standard cmos, because we get benefits of using the
> hypervisor's sense of time instead of the bare hardware, and allows for
> virtualization of the persistent clock.
>
> But the problem is that even if Dom0 tries to set the xen persistent
> clock, it doesn't actually update anything in the underlying hardware?
> So here you instead try to sync the underlying hardware cmos from the
> same Xen dom0 environment?

Yes.

> Honestly, it seems a little strange to me. If you're running as dom0,
> why does HYPERVISOR_dom0_op() not cause the hypervisor to set the cmos
> its virtualizing? This seems to mess with the proper virtualization
> layering.

As Jan says the hypervisor only drives a minimal set of hardware,
everything else is made accessible to dom0 for it to control.

I think this makes sense as it allows us to reuse the existing RTC
drivers etc. in the Linux kernel, instead of having to reimplement them
in the hypervisor.

David


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