Linux 4.14 Ensures The "Core Performance Boost" Bit Gets Set For AMD Ryzen CPUs

Recently making waves in our forums was talk of a kernel patch to address a case where the AMD CPB (Core Performance Boost) isn't being exposed by Ryzen processors. Here's more details on that and some benchmarks.

Being talked about recently is f7f3dc0: "CPUID Fn8000_0007_EDX[CPB] is wrongly 0 on models up to B1. But they do support CPB (AMD's Core Performance Boosting cpufreq CPU feature), so fix that."

Basically the bit for indicating Core Performance Boost support indicates it's disabled for Ryzen CPUs while it should be enabled for hitting the boost frequencies on these processors. This patch isn't for Linux 4.15 but was already merged as a fix for Linux 4.14.

I confirmed the change with a Ryzen 7 1800X that is stepping 1 and indicated from /proc/cpuinfo via the flags line you can look for the presence of "cpb." (For reference, EPYC is stepping 2 and thus unaffected by this error.) Booting to Linux 4.13, the CPB string wasn't shown but is in fact reported when booting to the Linux 4.14 kernel.

Via the Ryzen 7 1800X setup I then proceeded to run some benchmarks to see if the performance was any difference with the new kernel:





But with a range of tests run, there isn't any real performance change to find out of the variety of tests executed.