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

[Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>]

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?

> 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-requirements-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.

-boris


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