Revisiting Intel CPU bug workarounds with Linux Kernel 5.2

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The impact of all the workarounds for all the Intel security is not that bad when it comes to every-day workloads like kernel compilation if you risk leaving SMT on. Turning SMT off does have a very real and noticeable impact on performance. Leaving SMT on and sticking with the kernels default settings will only add about a minute to a half-an-hour long compile job compared to turning the workarounds off with mitigations=off

This test was done using a laptop with a Intel(R) Core(TM) i7-5500U CPU @ 2.40GHz which has bugs: cpu_meltdown spectre_v1 spectre_v2 spec_store_bypass l1tf mds identified and exposed by the kernel in /proc/cpuinfo

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This i7-5500U is a dual-core four-thread CPU. It is older Broadwell technology which means that the real difference in seconds is higher and more visible than it would be on a newer and faster CPU. Also, we don't have any newer and faster Intel CPUs anymore.

Timed-5.2-compilation.png

Compiling Linux Kernel 5.2 with GCC 9.1.1 using no kernel parameters and the default kernel settings, which in this kernel is equal to mitigations=auto according to admin-guide/kernel-parameters.txt, results in a compile-time of 28 minutes and 9 seconds.

Turning all mitigations=off for maximum risk, excitement and performance shaves kernel compile time on this system down to 27 minutes and 25 seconds. That's just 43 seconds on a half-an-hour compile job. It really isn't all that noticeable.

Using mitigations=auto and mds=full,nosmt for maximum security on this CPU increases the compile-time to 33 minutes and 16 seconds. That is almost six minutes slower than mitigations=off

The obvious conclusion is that there really isn't that much difference between mitigations=off and the default mitigations=auto setting. But there is a huge difference between leaving SMT on and mds=full,nosmt

The stock kernel defaults are probably fine unless you're a financial institution or a cloud provider. The risks entailed by leaving SMT on are much bigger if random people/customers are able to run whatever they want on a shared CPU. They are not that high on a casually used home laptop or a gaming rig with more RGB lights than a Christmas tree.


published 2019-07-09last edited 2019-07-09


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