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

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Message-ID: <56684595.4040402@oracle.com>
Date:	Wed, 9 Dec 2015 10:15:33 -0500
From:	Boris Ostrovsky <boris.ostrovsky@...cle.com>
To:	Sander Eikelenboom <linux@...elenboom.it>,
	Jan Beulich <JBeulich@...e.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 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.

-boris

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