On 08/10/2015 03:16 AM, tiger...@zhaoxin.com wrote:
> Hello, experts:
>
> I have a question about x2apic support in UEFI BIOS.
>
> Could UEFI BIOS determine switching to xapic or x2apic when handing off
> to OS loader?
>
> Some webpages said:
>
> If CPU number < 255, BIOS should set to xapic mode.
Yes, Intel's specs. say that if all threads' APIC IDs are < 255, then
the system should start in xapic mode. Otherwise it should start in
x2apic mode.
BIOSes which I've seen make this decision in CpuMpDxe, when determining
the APIC IDs for all the CPUs. I don't know how you'd do it later on,
during loader handoff... once the APs' APICs are initialized, you need
to be in x2apic mode in order to send interrupts to CPUs with APIC IDs
>= 255. So BIOS would have no way to wake up the high-numbered APs to
finish their initialization if CpuMpDxe didn't switch the BSP to x2apic
mode.
I'm not sure if OVMF's CpuMpDxe handles booting in x2apic mode or not.
I haven't looked.
--
Brian J. Johnson
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