linux-kernel - Re: Why Linux kernel forced to enter X2APIC mode( just because of booting cpu has supported x2apic) without depending on BIOS' setting in MSR->x2apic enablement bit ?

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Message-ID: <50D1E6ED.2070803@zytor.com>
Date:	Wed, 19 Dec 2012 08:10:21 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Yinghai Lu <yinghai@...nel.org>
CC:	"Zhang, Lin-Bao (Linux Kernel R&D)" <linbao.zhang@...com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"alan@...rguk.ukuu.org.uk" <alan@...rguk.ukuu.org.uk>,
	"mingo@...hat.com" <mingo@...hat.com>,
	"tglx@...utronix.de" <tglx@...utronix.de>,
	"x86@...nel.org" <x86@...nel.org>,
	"a.p.zijlstra@...llo.nl" <a.p.zijlstra@...llo.nl>,
	"jarkko.sakkinen@...el.com" <jarkko.sakkinen@...el.com>,
	"joerg.roedel@....com" <joerg.roedel@....com>,
	"agordeev@...hat.com" <agordeev@...hat.com>
Subject: Re: Why Linux kernel forced to enter X2APIC mode( just because of
 booting cpu has supported x2apic) without depending on BIOS' setting in MSR->x2apic
 enablement bit ?

On 12/18/2012 09:49 AM, Yinghai Lu wrote:
> 
> So you need to make cpuid show does not support x2apic --- check with
> intel they have way to do that.
> 

No, but there is a flag in ACPI that "this BIOS is broken with x2APIC".
Setting that flag as opposed to fixing the bugs, though is very much not
recommended, however; the lack of interrupt routing means there are ways
for a rogue device to compromise the security provided by VT-d, see:

http://www.invisiblethingslab.com/resources/2011/Software%20Attacks%20on%20Intel%20VT-d.pdf

	-hpa


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