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: <CAE9FiQVAPnc7P91AK8gdwgY-xFsXMM_MxYZAu2sRjsp3EdA96w@mail.gmail.com>
Date:	Wed, 19 Dec 2012 08:05:38 -0800
From:	Yinghai Lu <yinghai@...nel.org>
To:	"Zhang, Lin-Bao (Linux Kernel R&D)" <linbao.zhang@...com>
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 ?

On Wed, Dec 19, 2012 at 2:36 AM, Zhang, Lin-Bao (Linux Kernel R&D)
<linbao.zhang@...com> wrote:
> 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) ).

how does BIOS cleaim?

disable that in CPUID?

>
> If Linux don't need BIOS' help , how did linux kernel reach this action?

need DMAR table to enable intr remapping.
--- ioapic only have 8 bits for dst.
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