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

[Juergen Gross]

On 08/04/16 08:56, Luis R. Rodriguez wrote:
> On Thu, Apr 7, 2016 at 11:38 PM, Juergen Gross <jgross@xxxxxxxx> wrote:
>> On 08/04/16 08:29, Luis R. Rodriguez wrote:
>>> On Thu, Apr 7, 2016 at 10:18 PM, Juergen Gross <jgross@xxxxxxxx> wrote:
>>>> On 08/04/16 02:32, Luis R. Rodriguez wrote:
>>>>> 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?
>>
>> Don't try to guess.
> 
> I can only do that given there is nothing at all to tell me what to
> expect here with regards to RTC on Xen guest, if there is some
> documentation that could help with that please let me know.

Only Xen inernals. :-)

> 
>> In case you don't want to inject Xen internals here, just call a Xen
>> function to either return the correct value, or to set all structure
>> elements correctly.
> 
> I like the later as an option, in case there are further hardware
> subarch specific quirks which require internal logistics. What do
> others think?
> 
>> Thinking more about it: why not do that for all the subarchs?
> 
> I originally had went with that approach, but Ingo made the point that
> it would be best to instead move all quirk settings into one place.
> That lets a reader easily tell what is going on in one place, it also
> compartmentalizes the hardware subarch uses.

Okay. Another idea (not sure whether this is really a good one):

Add X86_SUBARCH_XEN_DOM0. As hardware_subarch is 32 bits wide I don't
think the number of subarchs is a scarce resource. :-)

I'd expect other quirks in future might have different settings for
domU and dom0, too.

>> You'd
>> have the specific settings where they belong: in a subarch specific
>> source. Just do the default settings in x86_early_init_platform_quirks()
>> and let the subarch functions set the non-default values.
> 
> This is a rather different approach than what I had originally tried.
> Bike shed thing -- someone just has to decide.
> 
> Left up to me, I kind of really like centralizing the quirk settings
> in one place approach as it means a reader can easily tell what's
> going on regardless of platform in one place for odd settings. I
> prefer this given that we *already* have the semantics over hardware
> subarch in a generalized fashion. We *do not* have semantics for dom0
> Vs domU -- if such a notion is generic to other virtualization

That's not carved in stone - see above. :-)

> environments it deserves consideration to new semantics to deal with
> that, otherwise the callback for handling further quirks is best, but
> I'd also highly discourage such callback to be used.


Juergen