[S] Change in coreboot[master]: include/cpu/x86/lapic.h: Add constraint for SMM

[Patrick Rudolph (Code Review)]

Attention is currently required from: Arthur Heymans, Jonathan Zhang, Ziang Wang, Paul Menzel, Christian Walter, Angel Pons.

Patrick Rudolph has posted comments on this change. ( https://review.coreboot.org/c/coreboot/+/74054 )

Change subject: include/cpu/x86/lapic.h: Add constraint for SMM
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Patch Set 2:

(1 comment)

File src/include/cpu/x86/lapic.h:

https://review.coreboot.org/c/coreboot/+/74054/comment/cbc8adea_46cec172 
PS2, Line 64: is_x2apic_mode
The whole function is wrong. It assumes that state of the apic, but that's not valid until MPinit has run.
When CONFIG_X2APIC_ONLY is selected and something is calling lapicid() before MPinit it'll cause an exception. As the exception handler also calls lapicid() it tripple faults.