linux-kernel - Re: [Xen-devel] [PATCH] x86: Xen PV guests don't have the rtc

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Date:	Wed, 09 Dec 2015 08:27:14 -0700
From:	"Jan Beulich" <JBeulich@...e.com>
To:	"Sander Eikelenboom" <linux@...elenboom.it>,
	"Boris Ostrovsky" <boris.ostrovsky@...cle.com>
Cc:	"David Vrabel" <david.vrabel@...rix.com>, <x86@...nel.org>,
	<tglx@...utronix.de>, <xen-devel@...ts.xenproject.org>,
	<mingo@...hat.com>, <vkuznets@...hat.com>,
	<linux-kernel@...r.kernel.org>, <stable@...r.kernel.org>,
	<hpa@...or.com>
Subject: Re: [Xen-devel] [PATCH] x86: Xen PV guests don't have the
 rtc_cmos platform device

>>> On 09.12.15 at 16:15, <boris.ostrovsky@...cle.com> wrote:
> On 12/09/2015 10:00 AM, Sander Eikelenboom wrote:
>> On 2015-12-09 15:42, Jan Beulich wrote:
>>>>>> On 09.12.15 at 15:32, <boris.ostrovsky@...cle.com> wrote:
>>>> --- a/arch/x86/kernel/rtc.c
>>>> +++ b/arch/x86/kernel/rtc.c
>>>> @@ -200,6 +200,9 @@ static __init int add_rtc_cmos(void)
>>>>      }
>>>>  #endif
>>>>
>>>> +    if (paravirt_enabled())
>>>> +        return -ENODEV;
>>>
>>> What about Xen Dom0?
>>>
>>> Jan
>>
>> Checked that in my testing and that still worked:
>> [   16.733837] rtc_cmos 00:02: RTC can wake from S4
>> [   16.734030] rtc_cmos 00:02: rtc core: registered rtc_cmos as rtc0
>> [   16.734087] rtc_cmos 00:02: alarms up to one month, y3k, 114 bytes 
>> nvram
>> [   17.760329] rtc_cmos 00:02: setting system clock to 2015-12-09 
>> 08:43:48 UTC (1449650628)
>>
>> and /dev/rtc and /dev/rtc0 both exist.
>>
>> But i don't know the nitty gritty details about why ...
> 
> 
> That's because it is discovered by ACPI earlier. I don't know though 
> whether we can always assume this will be the case.

I don't think we should - Dom0 should (device-wise) behave just
like a native kernel.

Jan

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