https://wiki.xenproject.org/wiki/Xen_Project_4.21_Feature_List
x86: New amd-cppc/amd-cppc-epp cpufreq driver.
On my system, the warning is about CPPC being available on the CPU, but disabled in the firmware.
There is a bit more information in the 4.21 press release
amd-cppc and amd-cppc-epp cpufreq drivers to enable fine-grained CPU performance scaling on AMD processors through Collaborative Processor Performance Control (CPPC)
To me, it sounds mostly like a power consumption improvement.
When running geekbench in a VM with all cores assigned, the benchmark score is close to an 9950X running bare-metal Linux, which is why I don’t think the powernow driver is a performance problem.