STIBP Patches Updated One Last Time Before Heading To Linux 4.20
Written by Michael Larabel in Linux Kernel on 25 November 2018 at 04:07 PM EST. 5 Comments
Thomas Gleixner on Sunday sent out the second version of the cleaned up patches around lowering the overhead of STIBP "Single Thread Indirect Branch Predictors" and the related IBPB "Indirect Branch Predictor Barrier" for Linux 4.20.

With Linux 4.20 STIBP got applied for all processes and that caused a major performance hit while now it's only on by default for SECCOMP processes or processes opting for it via the prctl interface. More details and my tests of these revised patches from just a few days ago can be found in Benchmarking The Work-In-Progress Spectre/STIBP Code On The Way For Linux 4.20.

Long story short, the patches clear up that very dramatic performance drop saw earlier in the Linux 4.20 cycle. STIBP had been back-ported to existing supported stable series, but has already been reverted due to the performance tax while these new patches may eventually work their way into those LTS trees.

With the second version of the patches sent out today, there were changes to address review comments, renaming of the command-line option to spectre_v2_user=, the options for conditional STIBP and IBPB always modes, and other minor code changes. The V2 patches can be found on the kernel mailing list. Linus Torvalds is already calling for some documentation changes with this issue having caused him some grief with the performance hit not having been properly communicated back during the Linux 4.20 merge window.

Gleixner anticipates these V2 patches to be "hopefully the final version", so ideally in the week ahead we'll see the code merged in time for Linux 4.20-rc5, which still gives it a few weeks of time for testing before the stable Linux 4.20 debut in late December or early January.
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