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

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Date:	Tue, 14 May 2013 11:03:53 -0700
From:	John Stultz <john.stultz@...aro.org>
To:	David Vrabel <david.vrabel@...rix.com>
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>
Subject: Re: [PATCH 3/3] x86/xen: sync the CMOS RTC as well as the Xen wallclock

On 05/14/2013 11:00 AM, David Vrabel wrote:
> On 14/05/13 18:24, 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).
>>>
>>> Signed-off-by: David Vrabel <david.vrabel@...rix.com>
>>> ---
>>>    arch/x86/xen/time.c |   11 ++++++++++-
>>>    1 files changed, 10 insertions(+), 1 deletions(-)
>>>
>>> diff --git a/arch/x86/xen/time.c b/arch/x86/xen/time.c
>>> index a1947ac..4656165 100644
>>> --- a/arch/x86/xen/time.c
>>> +++ b/arch/x86/xen/time.c
>>> @@ -14,6 +14,7 @@
>>>    #include <linux/kernel_stat.h>
>>>    #include <linux/math64.h>
>>>    #include <linux/gfp.h>
>>> +#include <linux/mc146818rtc.h>
>>>    
>>>    #include <asm/pvclock.h>
>>>    #include <asm/xen/hypervisor.h>
>>> @@ -199,17 +200,25 @@ static void xen_get_wallclock(struct timespec *now)
>>>    static int xen_set_wallclock(const struct timespec *now)
>>>    {
>>>    	struct xen_platform_op op;
>>> +	int ret;
>>>    
>>>    	/* do nothing for domU */
>>>    	if (!xen_initial_domain())
>>>    		return -1;
>>>    
>>> +	/* Set the Xen wallclock. */
>>>    	op.cmd = XENPF_settime;
>>>    	op.u.settime.secs = now->tv_sec;
>>>    	op.u.settime.nsecs = now->tv_nsec;
>>>    	op.u.settime.system_time = xen_clocksource_read();
>>>    
>>> -	return HYPERVISOR_dom0_op(&op);
>>> +	ret = HYPERVISOR_dom0_op(&op);
>>> +	if (ret)
>>> +		return ret;
>>> +
>>> +	/* Set the hardware RTC. */
>>> +	return mach_set_rtc_mmss(now);
>> Sorry, just noticed one more thing while applying this. Do all Xen
>> systems run on hardware that has the conventional CMOS clock?
>>
>> What happens if the bare-metal needs to use efi_set_rtc() or
>> vrtc_set_mmss() ?
> Ug. I hadn't considered that.  I'll have think some more on this.

Ok. I'll look forward to a future revision then and not queue this one.


Just FYI, here's what I rewrote the commit message to:

     x86/xen: Sync the CMOS RTC as well as the Xen wallclock

     Adjustments to Xen's persistent_clock via update_persistent_clock()
     don't actually persist, as the xen_set_walltime() just notifies
     other domN guests that it has been updated, and does not modify
     the underlying CMOS clock.

     Thus, this patch modifies xen_set_wallclock() so it will set
     the underlying CMOS clock when called from dom0, ensuring the
     persistent_clock will be correct on the next hardware boot.


I feel its a bit more clear, but feel free to tweak it as you see fit.

thanks
-john

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