[coreboot-gerrit] Change in coreboot[master]: cpu/x86/smm/smm_stub: Add x86_64 support

[Eugene Myers (Code Review)]

Eugene Myers has posted comments on this change. ( https://review.coreboot.org/c/coreboot/+/37392 )

Change subject: cpu/x86/smm/smm_stub: Add x86_64 support
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Patch Set 6:

Did a closer review and realized that this patch will break STM support.

If you turn on the STM, the build should fail because start32_offset is missing. The STM initialization functions use this value to create the entry point to start the SMI handler in 32-bit mode.

Secondly, the tss segment selector (0x20) has to be there because when the STM starts the SMI handler VM, the VMX startup checks the TSS selector for proper values.  Without it, the VM start will fail with an invalid guest state error.