Re: [PATCH v3 09/12] KVM: VMX: Remove vmx->current_tsc_ratio and decache_tsc_multiplier()

[Sean Christopherson]

On Tue, May 25, 2021, Stamatis, Ilias wrote:
> Hmm, this patch actually still removes the caching and introduces a small
> performance overhead. For example if neither L1 nor L2 are scaled it will
> still do a vmwrite for every L2 entry/write.

True, but there is an ocean of difference between the relative performance of
vmx_vcpu_load_vmcs() and a nested transition.  vmx_vcpu_load_vmcs() is also
called much more frequently.

> So do we want to get rid of decache_tsc_multiplier() but keep 
> vmx->current_tsc_ratio and do the check inside write_tsc_multiplier()? Or 
> alternatively delete vmx->current_tsc_ratio too and have 
> write_tsc_multiplier() receive 2 parameters, one of the old multiplier and 
> one of the new?

My vote is to kill it, eat the barely-noticeable perf hit on nVMX, and tackle
the aggressive VMCS shadowing in a separate series.