linux-kernel - Re: [PATCH 14/15] KVM: x86: Tie Intel and AMD behavior for MSR_TSC

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Date:   Mon, 10 May 2021 16:50:23 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Maxim Levitsky <mlevitsk@...hat.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, Xiaoyao Li <xiaoyao.li@...el.com>,
        Reiji Watanabe <reijiw@...gle.com>
Subject: Re: [PATCH 14/15] KVM: x86: Tie Intel and AMD behavior for
 MSR_TSC_AUX to guest CPU model

On Mon, May 10, 2021, Maxim Levitsky wrote:
> On Tue, 2021-05-04 at 10:17 -0700, Sean Christopherson wrote:
> > diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> > index de921935e8de..6c7c6a303cc5 100644
> > --- a/arch/x86/kvm/svm/svm.c
> > +++ b/arch/x86/kvm/svm/svm.c
> > @@ -2663,12 +2663,6 @@ static int svm_get_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
> >  			msr_info->data |= (u64)svm->sysenter_esp_hi << 32;
> >  		break;
> >  	case MSR_TSC_AUX:
> > -		if (tsc_aux_uret_slot < 0)
> > -			return 1;
> > -		if (!msr_info->host_initiated &&
> Not related to this patch, but I do wonder why do we need
> to always allow writing this msr if done by the host,
> since if neither RDTSPC nor RDPID are supported, the guest
> won't be able to read this msr at all.

It's an ordering thing and not specific to MSR_TSC_AUX.  Exempting host userspace
from guest CPUID checks allows userspace to set MSR state, e.g. during migration,
before setting the guest CPUID model.

> > -		    !guest_cpuid_has(vcpu, X86_FEATURE_RDTSCP) &&
> > -		    !guest_cpuid_has(vcpu, X86_FEATURE_RDPID))
> > -			return 1;
> >  		msr_info->data = svm->tsc_aux;
> >  		break;
> >  	/*