Re: [PATCH 04/13] cpufreq: Add Hardware P-State (HWP) driver

[Jan Beulich <jbeulich@xxxxxxxx>]

On 03.05.2021 21:28, Jason Andryuk wrote:
> If HWP Energy_Performance_Preference isn't supported, the code falls
> back to IA32_ENERGY_PERF_BIAS.  Right now, we don't check
> CPUID.06H:ECX.SETBH[bit 3] before using that MSR.  The SDM reads like
> it'll be available, and I assume it was available by the time Skylake
> introduced HWP.

Upon more detailed reading of the respective SDM sections, I only
see two options: Either you fail driver initialization if the bit
is clear, or you correctly deal with the bit being clear. If Xen
runs virtualized itself, the combination of CPUID bits set may
not match that of any bare metal hardware that exists.

Jan