Re: [PATCH v2 06/25] KVM: VMX: Defer enabling FRED MSRs save/load until after set CPUID

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On Wed, Feb 07, 2024, Xin Li wrote:
> Clear FRED VM entry/exit controls when initializing a vCPU, and set
> these controls only if FRED is enumerated after set CPUID.
> 
> FRED VM entry/exit controls need to be set to establish context
> sufficient to support FRED event delivery immediately after VM entry
> and exit.  However it is not required to save/load FRED MSRs for
> a non-FRED guest, which aren't supposed to access FRED MSRs.

Does this actually provide a measurable performance boost?  If not, just do the
unnecessary load/store on entry/exit.

Generally speaking, the only time KVM dynamically toggles entry/exit controls is
when KVM wants to run the guest with a host value, e.g. with the host's
PERF_GLOBAL_CTRL.




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