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

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Date:   Thu, 21 Jul 2022 15:01:01 +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 7/10/2022 9:38 PM, Maxim Levitsky wrote:
> On Fri, 2022-06-17 at 20:29 +0530, Shukla, Santosh wrote:
>>
>> 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.
> 
> How that is possible? if vGIF is disabled in L1, then L1 can't execute STGI/CLGI - 
> that means that the CPU can't update the V_NMI, as it never sees the STGI/CLGI
> beeing executed.
> 

If vGIF is disabled then HW will take the vnmi event at the boundary of vmrun instruction.

Thanks,
Santosh

> Best regards,
> 	Maxim Levitsky
> 
>>
>>>>
>>>>
>>>>> +
>>>>>         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
> 
>