Shulou(Shulou.com)06/02 Report--

OpenSUSE defaults to using IBRS as its Spectre Variant 2 mitigation method instead of Retpolines, which is one of the reasons why this release has slower out-of-the-box performance than other Linux distributions.

This opensuse-factory mailing list refers to the "huge single-core performance loss" of Lenovo laptops when using openSUSE. The single-threaded performance loss around Spectre Variant 2 mitigation measures is about 21%, which in itself is not surprising, as we have explained the performance cost of Spectre/Meltdown mitigation many times.

OpenSUSE's kernel is using IBRS (indirect branching limit speculation) and the latest Intel CPU microcode image, while most Linux distributions rely on Retpolines as the return trampolines. Compared with Retpolines, IBRS mitigation technology may lead to more performance loss. It is reported that when matched with the updated Intel CPU microcode, there will be a greater performance loss due to more stringent speculative behavior.

According to the mailing list discussion, switching to Retpolines to find problematic workloads can restore performance.

OpenSUSE users who want to use a non-default method can select it using spectre_v2=retpoline, a common kernel command-line argument that matches the behavior of most other Linux distributions kernels.

As for openSUSE changing their default values, at least from the previous mailing list discussion, their kernel engineers didn't seem interested in changing Spectre's mitigation defaults, but only blamed their problems for Intel's poor performance.

Some people also suggest that the openSUSE installer choose a switch in its installer to inform users of security and performance preferences, so as to better provide reasonable / sensible default settings, but so far, we have not seen any steps to achieve this. Although considering that some openSUSE conservative defaults do have a performance impact compared to most other Linux distributions.

The obstacles to the use of Retpolines in the past required support from the Retpolines compiler, but this support has been around for some time now. It is reported that Skylake has reported Retpolines problems in the past, but these problems also seem to have been solved.

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