linux-kernel - Re: [PATCH v4 04/14] x86/rtc: replace paravirt rtc check with platform legacy quirk

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Date:	Fri, 8 Apr 2016 08:25:33 -0400
From:	Boris Ostrovsky <boris.ostrovsky@...cle.com>
To:	"Luis R. Rodriguez" <mcgrof@...nel.org>,
	Juergen Gross <jgross@...e.com>
Cc:	Borislav Petkov <bp@...en8.de>, "H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	Rusty Russell <rusty@...tcorp.com.au>, X86 ML <x86@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Andy Lutomirski <luto@...capital.net>,
	David Vrabel <david.vrabel@...rix.com>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	"xen-devel@...ts.xensource.com" <xen-devel@...ts.xensource.com>,
	lguest@...ts.ozlabs.org,
	Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
	Joey Lee <jlee@...e.com>, Gary Lin <glin@...e.com>,
	Matt Fleming <matt@...eblueprint.co.uk>,
	Andrew Cooper <andrew.cooper3@...rix.com>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	Len Brown <lenb@...nel.org>,
	"Moore, Robert" <robert.moore@...el.com>,
	Lv Zheng <lv.zheng@...el.com>, Toshi Kani <toshi.kani@...com>,
	ACPI Devel Maling List <linux-acpi@...r.kernel.org>,
	kozerkov@...allels.com, Josh Triplett <josh@...htriplett.org>,
	Joerg Roedel <joro@...tes.org>
Subject: Re: [PATCH v4 04/14] x86/rtc: replace paravirt rtc check with
 platform legacy quirk

On 04/08/2016 02:29 AM, Luis R. Rodriguez wrote:
> On Thu, Apr 7, 2016 at 10:18 PM, Juergen Gross <jgross@...e.com> wrote:
>> On 08/04/16 02:32, Luis R. Rodriguez wrote:
>>> On Thu, Apr 07, 2016 at 08:55:54AM -0400, Boris Ostrovsky wrote:
>>>> On 04/06/2016 08:06 PM, Luis R. Rodriguez wrote:
>>>>> We have 4 types of x86 platforms that disable RTC:
>>>>>
>>>>>    * Intel MID
>>>>>    * Lguest - uses paravirt
>>>>>    * Xen dom-U - uses paravirt
>>>>>    * x86 on legacy systems annotated with an ACPI legacy flag
>>>>>
>>>>> We can consolidate all of these into a platform specific legacy
>>>>> quirk set early in boot through i386_start_kernel() and through
>>>>> x86_64_start_reservations(). This deals with the RTC quirks which
>>>>> we can rely on through the hardware subarch, the ACPI check can
>>>>> be dealt with separately.
>>>>>
>>>>> v2: split the subarch check from the ACPI check, clarify
>>>>>      on the ACPI change commit log why ordering works
>>>>>
>>>>> Suggested-by: Ingo Molnar <mingo@...nel.org>
>>>>> Signed-off-by: Luis R. Rodriguez <mcgrof@...nel.org>
>>> <-- snip -->
>>>
>>>>> diff --git a/arch/x86/kernel/platform-quirks.c b/arch/x86/kernel/platform-quirks.c
>>>>> new file mode 100644
>>>>> index 000000000000..1b114ac5996f
>>>>> --- /dev/null
>>>>> +++ b/arch/x86/kernel/platform-quirks.c
>>>>> @@ -0,0 +1,18 @@
>>>>> +#include <linux/kernel.h>
>>>>> +#include <linux/init.h>
>>>>> +
>>>>> +#include <asm/setup.h>
>>>>> +#include <asm/bios_ebda.h>
>>>>> +
>>>>> +void __init x86_early_init_platform_quirks(void)
>>>>> +{
>>>>> +   x86_platform.legacy.rtc = 1;
>>>>> +
>>>>> +   switch (boot_params.hdr.hardware_subarch) {
>>>>> +   case X86_SUBARCH_XEN:
>>>>> +   case X86_SUBARCH_LGUEST:
>>>>> +   case X86_SUBARCH_INTEL_MID:
>>>>> +           x86_platform.legacy.rtc = 0;
>>>>> +           break;
>>>>> +   }
>>>>> +}
>>>> What about Xen dom0 (aka initial domain)?
>>> Indeed, thanks for catching this, the hunk below removes the re-enablement of
>>> the the RTC for dom0:
>>>
>>>>> --- a/arch/x86/xen/enlighten.c
>>>>> +++ b/arch/x86/xen/enlighten.c
>>>>> @@ -1192,7 +1192,6 @@ static const struct pv_info xen_info __initconst = {
>>>>>   #ifdef CONFIG_X86_64
>>>>>      .extra_user_64bit_cs = FLAT_USER_CS64,
>>>>>   #endif
>>>>> -   .features = 0,
>>>>>      .name = "Xen",
>>>>>   };
>>>>> @@ -1525,8 +1524,6 @@ asmlinkage __visible void __init xen_start_kernel(void)
>>>>>      /* Install Xen paravirt ops */
>>>>>      pv_info = xen_info;
>>>>> -   if (xen_initial_domain())
>>>>> -           pv_info.features |= PV_SUPPORTED_RTC;
>>>>>      pv_init_ops = xen_init_ops;
>>>>>      if (!xen_pvh_domain()) {
>>>>>              pv_cpu_ops = xen_cpu_ops;
>>> This should then break dom0 unless of course you have the respective next
>>> patch applied and that disabled the RTC due to an ACPI setting on your
>>> platform. Juergen, can you check to see if that was the case for your
>>> testing platform on dom0 ?
>> Are you sure it would break?
> No, suspected that it should though.
>
>> Wouldn't it just fall back to another
>> clock source, e.g. hpet?
> I suppose so.
>
>> I looked into my test system: seems as if add_rtc_cmos() is returning
>> before the .legacy.rtc test.
> OK thanks...

It works because the clock must have been discovered by ACPI prior to 
add_rtc_cmos() call. It's PNP0b00 object, I believe. The rest of the 
routine is to handle the case when RTC is not found in ACPI tables for 
whatever reasons (I think).

That's why we added paravirt_has(RTC) --- dom0 should be able to handle 
such cases, just like bare metal.

-boris