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 06/06/2016 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) ..."

-boris

>
>> --- a/xen/common/libacpi/build.c
>> +++ b/xen/common/libacpi/build.c
>> @@ -555,6 +555,8 @@ void acpi_build_tables(struct acpi_config *config, 
>> unsigned long physical)
>>      fadt->x_dsdt = config->mem_ops.v2p(dsdt);
>>      fadt->firmware_ctrl   = config->mem_ops.v2p(facs);
>>      fadt->x_firmware_ctrl = config->mem_ops.v2p(facs);
>> +    if ( !(config->table_flags & ACPI_BUILD_IOAPIC) )
>> +        fadt->flags |= (1<<20); /* HW_REDUCED_ACPI */
> I think this would better be made a suitable #define in acpi2_0.h.
>
> Jan
>



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