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: <92645B27BF79D04FBD2B0F8494FFD0F91F2210@G1W3649.americas.hpqcorp.net>
Date:	Wed, 19 Dec 2012 10:13:50 +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 , 
	Thanks very much for your reply. I also checked some other previous emails about x2apic patches.

> From: yhlu.kernel@...il.com [mailto:yhlu.kernel@...il.com] On Behalf Of
> Yinghai Lu

I just worry about this case, 
> > b) If BIOS feel the system doesn't meet x2apic conditions , it will not set
> x2apic enablement bit in MSR ,and pass control to OS with xapic mode.
> 
> kernel will check if cpuid support x2apic, if it supports x2apic, it will check if
> DMAR/intr-remapping could be enabled, if so kernel will switch to x2apic.
> otherwise it will stay with xapic.
> 
> So you need to make cpuid show does not support x2apic --- check with intel
> they have way to do that.
My concerns is BIOS creates different ACPI tables in xAPIC mode and X2APIC mode. 
BIOS will auto detect if the system meets x2APIC condition , if no , it will not create right ACPI tables for this mode and don't set MSR bit.
For this case (BIOS didn't set MSR , but BSP supports x2apic ) , I want to know 
1, For Linux kernel, which ACPI tables will be used for x2apic and XAPIC ?
2, why Linux kernel will set x2apic enablement bit in CPU msr ? generally speaking , BIOS set it and OS get it.
If OS get it , who will read it ? BIOS ?  I consult BIOS designer , he said , their BIOS calling service(just like UEFI run time service)
will also judge MSR again , so I don't know why Linux kernel will set MSR bit , unless OS has special target . 

> or you can pass "nox2apic" in boot command line.
Yeah, I think this would be fine if we really want to use xAPIC mode instead of x2apic mode.
But I am puzzled why Linux kernel will force to enter x2apic just depends on BSP has supported x2apic ? 
"BSP supports x2apic" condition is enough for judging if OS can enable x2apic ? 


Thanks very much ! 

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