Speculation attacks against some high-performance processors rely
on being able to manipulate the branch predictor for a victim
context by executing aliasing branches in the attacker context.
Such attacks can be partially mitigated against by clearing
internal branch predictor state and limiting the prediction
logic in some situations.

This config option will take CPU-specific actions to harden
the branch predictor against aliasing attacks and may rely on
specific instruction sequences or control bits being set by
the system firmware.

If unsure, say Y.