linux-kernel - Re: [PATCH 4/7] KVM: SVM: Report NMI not allowed when Guest busy handling VNMI

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Date:   Tue, 07 Jun 2022 16:10:54 +0300
From:   Maxim Levitsky <mlevitsk@...hat.com>
To:     Santosh Shukla <santosh.shukla@....com>,
        Paolo Bonzini <pbonzini@...hat.com>
Cc:     Sean Christopherson <seanjc@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Tom Lendacky <thomas.lendacky@....com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 4/7] KVM: SVM: Report NMI not allowed when Guest busy
 handling VNMI

On Thu, 2022-06-02 at 19:56 +0530, Santosh Shukla wrote:
> In the VNMI case, Report NMI is not allowed when the processor set the
> V_NMI_MASK to 1 which means the Guest is busy handling VNMI.
> 
> Signed-off-by: Santosh Shukla <santosh.shukla@....com>
> ---
>  arch/x86/kvm/svm/svm.c | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> index d67a54517d95..a405e414cae4 100644
> --- a/arch/x86/kvm/svm/svm.c
> +++ b/arch/x86/kvm/svm/svm.c
> @@ -3483,6 +3483,9 @@ bool svm_nmi_blocked(struct kvm_vcpu *vcpu)
>         struct vmcb *vmcb = svm->vmcb;
>         bool ret;
>  
> +       if (is_vnmi_enabled(vmcb) && is_vnmi_mask_set(vmcb))
> +               return true;

How does this interact with GIF? if the guest does clgi, will the
CPU update the V_NMI_MASK on its own If vGIF is enabled?

What happens if vGIF is disabled and vNMI is enabled? KVM then intercepts
the stgi/clgi, and it should then update the V_NMI_MASK?




> +
>         if (!gif_set(svm))
>                 return true;
>  
> @@ -3618,6 +3621,9 @@ static void svm_enable_nmi_window(struct kvm_vcpu *vcpu)
>  {
>         struct vcpu_svm *svm = to_svm(vcpu);
>  
> +       if (is_vnmi_enabled(svm->vmcb) && is_vnmi_mask_set(svm->vmcb))
> +               return;

This might have hidden assumption that we will only enable NMI window when vNMI is masked.



> +
>         if ((vcpu->arch.hflags & (HF_NMI_MASK | HF_IRET_MASK)) == HF_NMI_MASK)
>                 return; /* IRET will cause a vm exit */
>  


Best regards,
	Maxim Levitsky