linux-kernel - Re: [PATCH v2] x86, vt-d: enable x2apic opt out

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Date:	Wed, 11 May 2011 16:07:10 +0100
From:	"Woodhouse, David" <david.woodhouse@...el.com>
To:	"Song, Youquan" <youquan.song@...el.com>
CC:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	"mingo@...e.hu" <mingo@...e.hu>,
	"tglx@...utronix.de" <tglx@...utronix.de>,
	"hpa@...or.com" <hpa@...or.com>,
	"hpa@...ux.intel.com" <hpa@...ux.intel.com>,
	"Kay, Allen M" <allen.m.kay@...el.com>,
	"Siddha, Suresh B" <suresh.b.siddha@...el.com>,
	"Liu, Kent" <kent.liu@...el.com>,
	Youquan Song <youquan.song@...ux.intel.com>
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 (http://download.intel.com/technology
> /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
David.Woodhouse@...el.com                              Intel Corporation

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