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: <92645B27BF79D04FBD2B0F8494FFD0F91F2222@G1W3649.americas.hpqcorp.net>
Date:	Wed, 19 Dec 2012 10:36:00 +0000
From:	"Zhang, Lin-Bao (Linux Kernel R&D)" <linbao.zhang@...com>
To:	Yinghai Lu <yinghai@...nel.org>
CC:	"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>,
	"hpa@...or.com" <hpa@...or.com>, "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 ?

Hi Yinghai , 
	
Actually ,my question like this: 
OS can really enable x2apic correctly without BIOS' help in this case : 
BIOS has claimed definitely it doesn't enable x2apic ,but OS feel it can enable x2apic(based on BSP supports x2apic) ).

If Linux don't need BIOS' help , how did linux kernel reach this action? 
I am reading this related code, I just see Linux kernel judge cpu_has_x2apic() ,if yes, it will set MSR, certainly , IR works right. 
We can think BIOS' supporting is not necessary ? 

Thanks very much . 
-- Bob(LinBao Zhang)
HP linux kernel enginner

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