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 ?

[Yinghai Lu]

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