Static calls in Linux 5.10

Dec 16, 2020

I was reading the Linux 5.10 release summary on KernelNewbies, and a section stood out to me:

1.6. Static calls for improved post-Spectre performance

Static calls are a replacement for global function pointers. They use code patching to allow direct calls to be used instead of indirect calls. They give the flexibility of function pointers, but with improved performance. This is especially important for cases where retpolines would otherwise be used, as retpolines can significantly impact performance.

I’ve spent a lot of time looking at the Linux kernel, but never directly at its indirect call setup or post-Spectre mitigations. These changes sound very cool, so I’m going to use this post to try and explain and understand them (both to myself and others).

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