Linux 6.5-rc4 Brings Change For Enabling STIBP On AMD Zen 4 Auto IBRS Systems

Last weekend I wrote about Zen 4's Automatic IBRS security feature needing STIBP enabled for protecting user-space processes. Single-Threaded Indirect Branch Predictors though haven't been enabled up to now with the Auto IBRS functionality on Linux. But the x86/urgent pull request sent out today ahead of the Linux 6.5-rc4 tagging makes that change.

AMD Automatic IBRS is akin to Intel eIBRS with this new feature being found on all AMD Zen 4 processors. Since Linux 6.3 there has been support for using Automatic IBRS by default on capable AMD systems but it didn't end up enabling STIBP for user processes.

The x86/urgent pull request sent to Linus Torvalds this morning now ensures STIBP gets flipped on when using Automatic IBRS. That is the main change as part of this week's x86/urgent material.