Messages in this thread | | | Date | Tue, 14 May 2013 10:55:33 +0100 | From | David Vrabel <> | Subject | Re: [PATCH 3/3] x86/xen: sync the CMOS RTC as well as the Xen wallclock |
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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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