Comment by amluto

You’re missing the big ones: both the interrupt delivery and the IRET (interrupt return) mechanisms use incredibly complicated data structures to determine what the new state should be. They need to dig around in the IDT, the GDT, the TSS and possibly the LDT to find all the register values they need to set, and they need to handle all kinds of backwards compatibility. And they “serialize”, which is an extra heavyweight fence, although that only likely accounts for a few hundred cycles in each direction.

Check out the pseudocode in the SDM — there are pages of it, and the pseudocode isn’t even complete.

FRED simplifies the state transitions such that the new state is mostly a foregone conclusion based on MSR contents.