linux-kernel - Re: [PATCH 3/3] x86/xen: sync the CMOS RTC as well as the Xen wallclock

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Message-ID: <51920A15.6050605@citrix.com>
Date:	Tue, 14 May 2013 10:55:33 +0100
From:	David Vrabel <david.vrabel@...rix.com>
To:	John Stultz <john.stultz@...aro.org>
CC:	"xen-devel@...ts.xen.org" <xen-devel@...ts.xen.org>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Jeremy Fitzhardinge <jeremy.fitzhardinge@...rix.com>
Subject: Re: [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@...rix.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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