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

[Juergen Gross]

On 08/04/16 09:36, Luis R. Rodriguez wrote:
> On Fri, Apr 8, 2016 at 12:13 AM, Juergen Gross <jgross@xxxxxxxx> wrote:
>> 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. :-)
> 
> Where can I look at this specifically on the Xen sources? No worries
> if you don't care -- as I don't really either.

Just look how xen_initial_domain() is defined. :-)

>>>> 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. :-)
> 
> This would mean bumping the x86 boot protocol, we shouldn't take that
> lightly, but given that in this case the new subarch would really only
> be set by the kernel (or future loaders for perhaps HVMLite) I'd think
> this is not such an intrusive alternative.

I think adding an own subarch for dom0 isn't that bad. It really is
different from domU as dom0 has per default access to the real hardware
(or at least to most of it).

>> I'd expect other quirks in future might have different settings for
>> domU and dom0, too.
> 
> Can you elaborate a bit more on this.

I guess we might want to add other quirks in order to switch on/off
more features instead of doing this based on #ifdef or environment
tests. I'm thinking of current use, not of HVMlite specific stuff.

> I realize we expect domU and
> dom0 on HVMLite in the future, would HVMLite use the subarch ? From
> the last discussions on the HVMLite thread Boris noted HVMLite would
> use the PC subarch -- how would we do dom0 Vs domU quirk management?

This would have to be settled. I think it might be a good idea to
initialize the quirks to their defaults statically in x86_init.c
and just modify some as needed for HVMlite on early boot (e.g. in
the HVMlite or EFI stub). This will enable us to either make use of
subarch or not for HVMlite, just what fits best.

> If it goes the EFI route, I gather Xen instead can use the EFI
> configuration tables for Xen specific tunings, however we may also
> need a generic x86 configuration table for generic quirks I think. We
> may need to provide a 1-1 mapping of these quirks there, if the
> subarch is not used.

The EFI stub can set the quirks just according to it's needs. Going
this route would _require_ HVMlite is using the PC subarch, though,
in order to avoid overwriting the quirks in
x86_early_init_platform_quirks() later.

> 
>>>> 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. :-)
> 
> Another subarch for Xen dom0 works well for me as well given the new
> subarch would just all set in the kernel. It does mean extending the
> x86 boot protocol though, and so for that I yield to hpa.

Fair enough.


Juergen