Re: [Xen-devel] [PATCH RFC 19/20] acpi: Set HW_REDUCED_ACPI in FADT if IOAPIC is not supported

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 07.06.16 at 17:17, <boris.ostrovsky@xxxxxxxxxx> wrote:
> On 06/07/2016 10:12 AM, Jan Beulich wrote:
>>>>> On 07.06.16 at 16:02, <boris.ostrovsky@xxxxxxxxxx> wrote:
>>> On 06/07/2016 02:06 AM, Jan Beulich wrote:
>>>>>>> On 06.06.16 at 19:31, <boris.ostrovsky@xxxxxxxxxx> wrote:
>>>>> On 06/06/2016 09:38 AM, Jan Beulich wrote:
>>>>>>>>> On 06.04.16 at 03:25, <boris.ostrovsky@xxxxxxxxxx> wrote:
>>>>>>> With this flags set guests will not try to set up SCI.
>>>>>> I've just read through the respective ACPI spec section again, and
>>>>>> I couldn't find a reference to SCI from there ("Hardware-Reduced
>>>>>> ACPI"). Can you clarify this connection please. Also there are other
>>>>>> consequences of setting that flag, so in order to understand the
>>>>>> reasons behind this change in case of future problems I think the
>>>>>> description here will need to be significantly extended, despite the
>>>>>> change being so small.
>>>>> My understanding is that hardware-reduced platforms don't use ACPI
>>>>> Platform Event Model (Sec. 4.1.1) and that model requires SCI (and vice
>>>>> versa --- SCI is present when ACPI Platform Event Model is in use). The
>>>>> (somewhat indirect) evidence of this is in section 4.6 "The ACPI
>>>>> Hardware Model" where is says: "In the ACPI Legacy state, the ACPI event
>>>>> model is disabled (no SCIs are generated) ..."
>>>> In the sum of all the non-explicit wording I can only convince myself
>>>> that SCI is a prereq for the event model. Yet I could see this being
>>>> an if-and-only-if, just that I couldn't find any place saying so.
>>> Not sure how I should interpret this: do you (reluctantly, possibly)
>>> agree that we can use HW-reduced flag to indicate that SCI is not there?
>> I really think we need to get confirmation on this from ACPI folks.
> 
> Who should those people be? linux-acpi?

That may yield valuable, but not dependable input. I'd rather think of
folks actually working on / contributing to the spec. I'm sure Intel can
name a few of their employees ...

>> And I think (and I said so before) we need to understand all the
>> other implications from setting that flag (i.e. we _cannot_ use this
>> flag _just_ to indicate there's no SCI).
> 
> FWIW, the Microsoft's reading is
> https://msdn.microsoft.com/en-us/windows/hardware/drivers/bringup/hardware-req
>  
> uirements-for-soc-based-platforms
> 
> ACPI fixed hardware features such as the following are not required:
>     Power Management (PM) timer
>     Real Time Clock (RTC) wake alarm
>     System Control Interrupt (SCI)
>     Fixed Hardware register set (PMx_* event/control/status registers)
>     GPE block registers (GPEx_* event/control/status registers)
>     Embedded controller
> 
> Also, from ACPICA perpective, HW-reduced mode appears to be the only way
> to prevent initialization of SCI.

Well, we could of course start out with HW-reduced mode, but we'd
then need to settle on all aspects before any of this becomes fully
supported.

Jan


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