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

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Date:   Fri, 17 Jun 2022 20:29:42 +0530
From:   "Shukla, Santosh" <santosh.shukla@....com>
To:     Maxim Levitsky <mlevitsk@...hat.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 6/7/2022 6:42 PM, Maxim Levitsky wrote:
> On Tue, 2022-06-07 at 16:10 +0300, Maxim Levitsky wrote:
>> 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?
>>
Yes.

>> 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?
>>
No.

For both case - HW takes the V_NMI event at the boundary of VMRUN instruction.

>>
>>
>>
>>> +
>>>         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.
> 
> Also what if vNMI is already pending?
> 
If V_NMI_MASK set, that means V_NMI is pending, if so then inject another V_NMI in next VMRUN.

Thanks,
Santosh

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