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

[Luis R. Rodriguez]

On Thu, Apr 7, 2016 at 10:18 PM, Juergen Gross <jgross@xxxxxxxx> 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@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?

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...

>> 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() ?

Because we don't want to sprinkle Xen specific code outside of Xen
code. What do you think about the second possibility I listed?
Otherwise, any other ideas?

  Luis