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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> 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.

Jan


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