Re: [RFC] KVM: x86: Give host userspace control for MSR_RAPL_POWER_UNIT and MSR_PKG_POWER_STATUS

[Xiaoyao Li <xiaoyao.li@intel.com>]

On 1/18/2023 10:21 PM, Anthony Harivel wrote:
> Allow userspace to update the MSR_RAPL_POWER_UNIT and
> MSR_PKG_POWER_STATUS powercap registers. By default, these MSRs still
> return 0.
> 
> This enables VMMs running on top of KVM with access to energy metrics
> like /sys/devices/virtual/powercap/*/*/energy_uj to compute VMs power
> values in proportion with other metrics (e.g. CPU %guest, steal time,
> etc.) and periodically update the MSRs with ioctl KVM_SET_MSRS so that
> the guest OS can consume them using power metering tools.
> 
> Cc: Paolo Bonzini <pbonzini@redhat.com>
> Cc: Christophe Fontaine <cfontain@redhat.com>
> Signed-off-by: Anthony Harivel <aharivel@redhat.com>
> ---
> 
> Notes:
>      The main goal of this patch is to bring a first step to give energy
>      awareness to VMs.
>      
>      As of today, KVM always report 0 in these MSRs since the entire host
>      power consumption needs to be hidden from the guests. However, there is
>      no fallback mechanism for VMs to measure their power usage.
>      
>      The idea is to let the VMMs running on top of KVM periodically update
>      those MSRs with representative values of the VM's power consumption.
>      
>      If this solution is accepted, VMMs like QEMU will need to be patched to
>      set proper values in these registers and enable power metering in
>      guests.
>      
>      I am submitting this as an RFC to get input/feedback from a broader
>      audience who may be aware of potential side effects of such a mechanism.

Set aside how user space VMM emulate these 2 MSRs correctly, it can 
request the MSR READ to exit to user space via KVM_X86_SET_MSR_FILTER. 
So user space VMM can just enable the read filter of these 2 MSRs and 
provide the emulation itself.