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 ?

[Zhang, Lin-Bao (Linux Kernel R&D)]

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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