Phoronix reports that a new bug fix for AMD Zen 4 chips was recently implemented into Linux version 6.6. The fix rectifies an issue surrounding undefined instruction exceptions potentially occurring on Zen 4 chips when the Single Threaded Indirect Branch Predictors (STIBP) are disabled. The bug, Erratum #1485, will also be back-ported to existing stable Linux kernels.
STIBP is a branch prediction feature in Zen 4 that restricts branch prediction sharing to one physical core and any logical cores used with the physical core (i.e., SMT/HyperThreading). STIBP is integrated into Zen 4's default security mitigations, specifically as part of the Spectre Variant Two protections.
By default, Zen 4 chips will automatically enable this feature since it is vital to the CPU's security systems. But, like other similar CPU security mitigations, enabling STIBP can incur a performance penalty. As a result, users can disable STIBP to regain performance when security is less of a concern.
Phoronix reports that this STIBP fix will also make its way to previous stable kernel versions but did not confirm when that will happen. There's a chance other operating systems could also be affected, but we haven't heard of any serious issues so far in Windows.
