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

[Juergen Gross <jgross@xxxxxxxx>]

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@xxxxxxxxxx>
>>> Signed-off-by: Luis R. Rodriguez <mcgrof@xxxxxxxxxx>
> 
> <-- 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? Wouldn't it just fall back to another
clock source, e.g. hpet?

I looked into my test system: seems as if add_rtc_cmos() is returning
before the .legacy.rtc test.

> This highlights a semantic gap issue. From a quick cursory review, I think
> we can address this temporarily by just using a check:
> 
> 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;
> +             if (x86_init.mpparse.get_smp_config != x86_init_uint_noop)
> +                     x86_platform.legacy.rtc = 0;

No! Why don't you just use the explicit test xen_initial_domain() ?


Juergen


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