From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756964Ab3ENJzh (ORCPT ); Tue, 14 May 2013 05:55:37 -0400 Received: from smtp.citrix.com ([66.165.176.89]:46954 "EHLO SMTP.CITRIX.COM" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756481Ab3ENJzg (ORCPT ); Tue, 14 May 2013 05:55:36 -0400 X-IronPort-AV: E=Sophos;i="4.87,669,1363132800"; d="scan'208";a="25062326" Message-ID: <51920A15.6050605@citrix.com> Date: Tue, 14 May 2013 10:55:33 +0100 From: David Vrabel User-Agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv:1.9.1.16) Gecko/20120428 Iceowl/1.0b1 Icedove/3.0.11 MIME-Version: 1.0 To: John Stultz CC: "xen-devel@lists.xen.org" , Konrad Rzeszutek Wilk , Thomas Gleixner , "linux-kernel@vger.kernel.org" , Jeremy Fitzhardinge Subject: Re: [PATCH 3/3] x86/xen: sync the CMOS RTC as well as the Xen wallclock References: <1368467768-2316-1-git-send-email-david.vrabel@citrix.com> <1368467768-2316-4-git-send-email-david.vrabel@citrix.com> <51918ABB.5030501@linaro.org> In-Reply-To: <51918ABB.5030501@linaro.org> Content-Type: text/plain; charset="ISO-8859-1" Content-Transfer-Encoding: 7bit X-Originating-IP: [10.80.2.76] Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 14/05/13 01:52, John Stultz wrote: > On 05/13/2013 10:56 AM, David Vrabel wrote: >> From: David Vrabel >> >> 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