linux-kernel - Re: [PATCH] KVM: SVM: Flush Hyper-V TLB when required

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Message-ID: <ZBs0fTo72LjnR22r@google.com>
Date:   Wed, 22 Mar 2023 10:01:49 -0700
From:   Sean Christopherson <seanjc@...gle.com>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>
Cc:     Jeremi Piotrowski <jpiotrowski@...ux.microsoft.com>,
        linux-kernel@...r.kernel.org, Paolo Bonzini <pbonzini@...hat.com>,
        kvm@...r.kernel.org, Tianyu Lan <ltykernel@...il.com>,
        Michael Kelley <mikelley@...rosoft.com>, stable@...r.kernel.org
Subject: Re: [PATCH] KVM: SVM: Flush Hyper-V TLB when required

On Wed, Mar 22, 2023, Vitaly Kuznetsov wrote:
> Sean Christopherson <seanjc@...gle.com> writes:
> > diff --git a/arch/x86/kvm/svm/svm_onhyperv.h b/arch/x86/kvm/svm/svm_onhyperv.h
> > index cff838f15db5..d91e019fb7da 100644
> > --- a/arch/x86/kvm/svm/svm_onhyperv.h
> > +++ b/arch/x86/kvm/svm/svm_onhyperv.h
> > @@ -15,6 +15,13 @@ static struct kvm_x86_ops svm_x86_ops;
> >  
> >  int svm_hv_enable_l2_tlb_flush(struct kvm_vcpu *vcpu);
> >  
> > +static inline bool svm_hv_is_enlightened_tlb_enabled(struct kvm_vcpu *vcpu)
> > +{
> > +	struct hv_vmcb_enlightenments *hve = &to_svm(vcpu)->vmcb->control.hv_enlightenments;
> > +
> > +	return !!hve->hv_enlightenments_control.enlightened_npt_tlb;
> 
> In theory, we should not look at Hyper-V enlightenments in VMCB control
> just because our kernel has CONFIG_HYPERV enabled.

Oooh, right, because hv_enlightenments uses software reserved bits, and in theory
KVM could be running on a different hypervisor that uses those bits for something
completely different.

> I'd suggest we add a
> real check that we're running on Hyper-V and we can do it the same way
> it is done in svm_hv_hardware_setup()/svm_hv_init_vmcb():
> 
> 	return (ms_hyperv.nested_features & HV_X64_NESTED_ENLIGHTENED_TLB)
> 		&& !!hve->hv_enlightenments_control.enlightened_npt_tlb;

Jeremi, if you grab this, can you put the && on the previous line?  I.e.

	return (ms_hyperv.nested_features & HV_X64_NESTED_ENLIGHTENED_TLB) &&
	       !!hve->hv_enlightenments_control.enlightened_npt_tlb;