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Date:	Wed, 11 May 2011 16:07:10 +0100
From:	"Woodhouse, David" <>
To:	"Song, Youquan" <>
CC:	"" <>,
	"" <>,
	"" <>,
	"" <>,
	"" <>,
	"" <>,
	"Kay, Allen M" <>,
	"Siddha, Suresh B" <>,
	"Liu, Kent" <>,
	Youquan Song <>
Subject: Re: [PATCH v2] x86, vt-d: enable x2apic opt out

On Thu, 2011-04-14 at 08:06 +0100, Song, Youquan wrote:
> New version of VT-d2 specification (
> /computing/vptech/Intel(r)_VT_for_Direct_IO.pdf) includes a new feature that 
> provide firmware a way to request system software to opt out of enable x2APIC
> mode. DMAR ACPI table newly define flags.1 bit: x2APIC_OPT_OUT which is set to
> request System software opt out xAPIC mode if flags.0 bit:INTR_REMAP is also
> set.
> This patch enable the feature. Also re-define x2apic_supported() to address
> platform x2apic support needs 1)processor has x2apic capability 2)interrupt
> remapping support 3)firmware does not request opt-out. 

Given that x2apic is *required* to be safe from irq injection tricks,
why would we ever want to manually disable it?

Is this just a workaround for a crappy BIOS? What is the *actual* reason
for wanting to disable x2apic?

David Woodhouse                            Open Source Technology Centre                              Intel Corporation

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