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 19:34:03 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     "Stamatis, Ilias" <ilstam@...zon.com>,
        "mlevitsk@...hat.com" <mlevitsk@...hat.com>,
        "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>,
        "joro@...tes.org" <joro@...tes.org>,
        "mtosatti@...hat.com" <mtosatti@...hat.com>,
        "zamsden@...il.com" <zamsden@...il.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 25/05/21 18:34, Sean Christopherson wrote:
>> I actually like the idea of storing the expected value in kvm_vcpu and the
>> current value in loaded_vmcs.  We might use it for other things such as
>> reload_vmcs01_apic_access_page perhaps.
> I'm not necessarily opposed to aggressively shadowing the VMCS, but if we go
> that route then it should be a standalone series that implements a framework
> that can be easily extended to arbitrary fields.  Adding fields to loaded_vmcs
> one at a time will be tedious and error prone.  E.g. what makes TSC_MULTIPLIER
> more special than TSC_OFFSET, GUEST_IA32_PAT, GUEST_IA32_DEBUGCTL, GUEST_BNDCFGS,
> and other number of fields that are likely to persist for a given vmcs02?

That it can be changed via ioctls in a way that affects both vmcs01 and 
vmcs02.  So TSC_MULTIPLIER is in the same boat as TSC_OFFSET, which I 
agree we should shadow more aggressively, but the others are different.

Paolo