Another Potential Performance Optimization For KPTI Meltdown Mitigation

Now that the dust is beginning to settle around the Meltdown and Spectre mitigation techniques on the major operating systems, in the weeks and months ahead we are likely to see more performance optimizations come to help offset the performance penalties incurred by mitigations like kernel page table isolation (KPTI) and Retpolines. This week a new patch series was published that may help with KPTI performance.

Intel developer Dave Hansen discovered that the back when KPTI was known as KAISER allowed the user/kernel shared areas to be marked global that would reduce the TLB overhead. But with all the code churn and it transitioning to page table isolation, that code got dropped.

Hansen posted a set of 10 patches on Thursday for bringing back global pages for shared areas with the x86/pti code. With this the code avoids unnecessary TLB misses.

No performance reports were provided but will be interesting to see what impact if anything measurable it will have on real-world workloads that were affected by KPTI. The patches can be found on the kernel mailing list and will be tested in our next Spectre/Meltdown benchmarking roundup.