linux-kernel - Re: [PATCH v3 09/12] KVM: VMX: Remove vmx->current_tsc_ratio and decache_tsc

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Date:   Tue, 25 May 2021 23:35:03 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     "Stamatis, Ilias" <ilstam@...zon.com>
Cc:     "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "jmattson@...gle.com" <jmattson@...gle.com>,
        "Woodhouse, David" <dwmw@...zon.co.uk>,
        "vkuznets@...hat.com" <vkuznets@...hat.com>,
        "mtosatti@...hat.com" <mtosatti@...hat.com>,
        "joro@...tes.org" <joro@...tes.org>,
        "zamsden@...il.com" <zamsden@...il.com>,
        "mlevitsk@...hat.com" <mlevitsk@...hat.com>,
        "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "wanpengli@...cent.com" <wanpengli@...cent.com>
Subject: Re: [PATCH v3 09/12] KVM: VMX: Remove vmx->current_tsc_ratio and
 decache_tsc_multiplier()

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.