The Linux kernel since last year has mistakenly left systems relying on the original Indirect Branch Restricted Speculation (IBRS) for Spectre V2 mitigation without Single Threaded Indirect Branch Predictor (STIBP) coverage for cross-HyperThread dealing with this Spectre vulnerability. There is a patch underway that is resolving this issue for Intel Skylake era systems.

Since a change merged last June and being mainlined in Linux 5.19-rc2, there is no STIBP support when IBRS (the original, not to be confused with Intel eIBRS) is engaged. 

So in order to protect user-space threads with STIBP, this patch by Google engineer KP Singh is under review to allow having STIBP enabled with the "legacy" IBRS. Again, this just affects older processors relying on plain IBRS like those from Skylake/Skylake-derived designs. The patch is marked already for back-porting to stable Linux kernel series as well once its mainlined.

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