[edk2] question about x2apic support

[T***@zhaoxin.com]

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.

Best wishes,

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[Brian J. Johnson]

Post by T***@zhaoxin.com
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?
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

Post by T***@zhaoxin.com
= 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.

[Laszlo Ersek]

Post by Brian J. Johnson

Post by T***@zhaoxin.com
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?
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

Post by T***@zhaoxin.com
= 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.

OVMF resolves LocalApicLib to

UefiCpuPkg/Library/BaseXApicLib

not

UefiCpuPkg/Library/BaseXApicX2ApicLib

The former's INF file says "The Local Apic library supports xAPIC mode
only". The latter's says, "The Local Apic library supports x2APIC
capable processors which have xAPIC and x2APIC modes."

FWIW,

- KVM guests on x86 don't seem to support more than 255:
arch/x86/include/asm/kvm_host.h:#define KVM_MAX_VCPUS 255

- the count for fullvirt Xen guests on x86 appears to be 128:
xen/include/public/hvm/hvm_info_table.h:#define HVM_MAX_VCPUS 128

So I think OVMF should be alright with xAPIC only.

Thanks!
Laszlo

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