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

From: Yinghai Lu
Date: Thu Dec 20 2012 - 11:46:59 EST

On Thu, Dec 20, 2012 at 2:38 AM, Zhang, Lin-Bao (Linux Kernel R&D)
<linbao.zhang@xxxxxx> wrote:
> I think current core argument is our BIOS did following judgement ,
> but Linux kernel did not(Linux seems just judge BSP supports x2apic mode or not by CPUID method)

kernel checks that, if the BSP in x2apic mode, kernel will check if
intr-remapping can be enabled, otherwise
it will switch back to xapic mode.

> Our BSP really supports x2apic mode, however,as this spec, our BIOS think some of configuration of server
> could not meet x2apic conditions ,So didn't place local xAPICs into x2apic mode.
> But linux kernel indeed enable x2apic mode(just believe BSP, didn't do above judgement) . so this is the conflict.

so your BIOS is wrong, it does not enable intr-remapping.

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