Re: [PATCH] KVM: VMX: CPU frequency scaling for intel x86_64 KVM guests

[Peter Zijlstra <peterz@infradead.org>]

On Tue, May 31, 2022 at 10:59:25AM +0000, Jack Allister wrote:
> A VMM can control a vCPU's CPU frequency by interfacing with KVM via
> the vCPU file descriptor to enable/set CPU frequency scaling for a
> guest. Instead of creating a separate IOCTL to this this, KVM capabil-
> ities are extended to include a capability called
> KVM_CAP_CPU_FREQ_SCALING.
> 
> A generic set_cpu_freq interface is added to kvm_x86_ops
> to allow for architecture (AMD/Intel) independent CPU frequency
> scaling setting.
> 
> For Intel platforms, Hardware-Controlled Performance States (HWP) are
> used to implement CPU scaling within the guest. Further information on
> this mechanism can be seen in Intel SDM Vol 3B (section 14.4). The CPU
> frequency is set as soon as this function is called and is kept running
> until explicitly reset or set again.
> 
> Currently the AMD frequency setting interface is left unimplemented.
> 
> Please note that CPU frequency scaling will have an effect on host
> processing in it's current form. To change back to full performance
> when running in host context an IOCTL with a frequency value of 0
> is needed to run back at uncapped speed.

Nowhere does this explain *WHY* we would want to do this?