CPU

Microcode update

Use retpoline

IBRS Dynamic

IBRS Always On

non-Intel

?

Yes, default to conservative?

?

?

All Current Intel CPUs

Yes

Yes, Default 

If IBRS available, users can opt-in

Opt-in if more paranoid

Future Work

N/A

Not needed, but not harmful

Not recommended

Default if CPU enumerates “cheaper” IBRS