linux-kernel - Re: [PATCH RFC] x86/acpi: don't ignore I/O APICs just because there's no local APIC

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Date:	Thu, 18 Jun 2009 13:39:53 -0700
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Jeremy Fitzhardinge <jeremy@...p.org>
Cc:	Len Brown <lenb@...nel.org>, Ingo Molnar <mingo@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>,
	the arch/x86 maintainers <x86@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Xen-devel <xen-devel@...ts.xensource.com>
Subject: Re: [PATCH RFC] x86/acpi: don't ignore I/O APICs just because there's no local APIC

Jeremy Fitzhardinge <jeremy@...p.org> writes:

> On 06/18/09 12:27, Eric W. Biederman wrote:
>>> The only actual exception I know of is Xen's replacement of the physical
>>> local APIC with a paravirtualized interrupt interface.
>>>     
>>
>> No one ever has.  Xen doesn't have I/O APICs either.  Not in any real
>> sense.  Xen just has devices that looking like I/O apics if you don't
>> look close.
>>   
>
> Well, if acpi_pci_irq_lookup() and friends return the right things
> without having parsed the MADT and set up the secondary state, then we
> should be fine either way.
>
> acpi_irq_model gets tested in all sorts of random places, so I wonder if
> we'll need to set it to ACPI_IRQ_MODEL_IOAPIC (or something else?) to
> make things work properly.

And this is where things get interesting.  Xen strictly speaking has
already made that decision.  Unless you support non APIC mode it
should always be ACPI_IRQ_MODEL_IOAPIC.

But Xen runs the hardware so Xen knows, and Xen should be running
all of the acpi and what not to make it happen.

> Hm, and principle we just get the SCI gsi from the FADT, but there's all
> that other mucking about with it in the MADT processing... Wonder what
> needs to happen there...

Good question.  What does the domU case do?

Eric

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