linux-kernel - Re: [PATCHv3 5/8] KVM: SVM: Add VNMI support in inject

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Date:   Fri, 26 Aug 2022 21:56:55 +0530
From:   "Shukla, Santosh" <santosh.shukla@....com>
To:     "Maciej S. Szmigiero" <mail@...iej.szmigiero.name>,
        "Shukla, Santosh" <santosh.shukla@....com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        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, mlevitsk@...hat.com
Subject: Re: [PATCHv3 5/8] KVM: SVM: Add VNMI support in inject_nmi

On 8/26/2022 5:50 PM, Maciej S. Szmigiero wrote:
> On 26.08.2022 11:35, Shukla, Santosh wrote:
>> On 8/25/2022 7:46 PM, Maciej S. Szmigiero wrote:
>>> On 25.08.2022 16:05, Shukla, Santosh wrote:
>>>> On 8/25/2022 6:15 PM, Maciej S. Szmigiero wrote:
>>>>> On 25.08.2022 12:56, Shukla, Santosh wrote:
>>>>>> On 8/24/2022 6:26 PM, Maciej S. Szmigiero wrote:
>>>>>>> On 24.08.2022 14:13, Shukla, Santosh wrote:
>>>>>>>> Hi Maciej,
>>>>>>>>
>>>>>>>> On 8/11/2022 2:54 AM, Maciej S. Szmigiero wrote:
>>>>>>>>> On 10.08.2022 08:12, Santosh Shukla wrote:
>>>>>>>>>> Inject the NMI by setting V_NMI in the VMCB interrupt control. processor
>>>>>>>>>> will clear V_NMI to acknowledge processing has started and will keep the
>>>>>>>>>> V_NMI_MASK set until the processor is done with processing the NMI event.
>>>>>>>>>>
>>>>>>>>>> Signed-off-by: Santosh Shukla <santosh.shukla@....com>
>>>>>>>>>> ---
>>>>>>>>>> v3:
>>>>>>>>>> - Removed WARN_ON check.
>>>>>>>>>>
>>>>>>>>>> v2:
>>>>>>>>>> - Added WARN_ON check for vnmi pending.
>>>>>>>>>> - use `get_vnmi_vmcb` to get correct vmcb so to inject vnmi.
>>>>>>>>>>
>>>>>>>>>>       arch/x86/kvm/svm/svm.c | 7 +++++++
>>>>>>>>>>       1 file changed, 7 insertions(+)
>>>>>>>>>>
>>>>>>>>>> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
>>>>>>>>>> index e260e8cb0c81..8c4098b8a63e 100644
>>>>>>>>>> --- a/arch/x86/kvm/svm/svm.c
>>>>>>>>>> +++ b/arch/x86/kvm/svm/svm.c
>>>>>>>>>> @@ -3479,7 +3479,14 @@ static void pre_svm_run(struct kvm_vcpu *vcpu)
>>>>>>>>>>       static void svm_inject_nmi(struct kvm_vcpu *vcpu)
>>>>>>>>>>       {
>>>>>>>>>>           struct vcpu_svm *svm = to_svm(vcpu);
>>>>>>>>>> +    struct vmcb *vmcb = NULL;
>>>>>>>>>>       +    if (is_vnmi_enabled(svm)) {
>>>>>>>>>
>>>>>>>>> I guess this should be "is_vnmi_enabled(svm) && !svm->nmi_l1_to_l2"
>>>>>>>>> since if nmi_l1_to_l2 is true then the NMI to be injected originally
>>>>>>>>> comes from L1's VMCB12 EVENTINJ field.
>>>>>>>>>
>>>>>>>>
>>>>>>>> Not sure if I understood the case fully.. so trying to sketch scenario here -
>>>>>>>> if nmi_l1_to_l2 is true then event is coming from EVTINJ. .which could
>>>>>>>> be one of following case -
>>>>>>>> 1) L0 (vnmi enabled) and L1 (vnmi disabled)
>>>>>>>
>>>>>>> As far as I can see in this case:
>>>>>>> is_vnmi_enabled() returns whether VMCB02's int_ctl has V_NMI_ENABLE bit set.
>>>>>>>
>>>>>>
>>>>>> For L1 with vnmi disabled case - is_vnmi_enabled()->get_vnmi_vmcb() will return false so the
>>>>>> execution path will opt EVTINJ model for re-injection.
>>>>>
>>>>> I guess by "get_vnmi_vmcb() will return false" you mean it will return NULL,
>>>>> since this function returns a pointer, not a bool.
>>>>>
>>>>
>>>> Yes, I meant is_vnmi_enabled() will return false if vnmi param is unset.
>>>>
>>>>> I can't see however, how this will happen:
>>>>>> static inline struct vmcb *get_vnmi_vmcb(struct vcpu_svm *svm)
>>>>>> {
>>>>>>       if (!vnmi)
>>>>>>           return NULL;
>>>>>           ^ "vnmi" variable controls whether L0 uses vNMI,
>>>>>          so this variable is true in our case
>>>>>
>>>>
>>>> No.
>>>>
>>>> In L1 case (vnmi disabled) - vnmi param will be false.
>>>
>>> Perhaps there was a misunderstanding here - the case here
>>> isn't the code under discussion running as L1, but as L0
>>> where L1 not using vNMI - L1 here can be an old version of KVM,
>>> or Hyper-V, or any other hypervisor.
>>>
>>
>> Ok.
>>
>>> In this case L0 is re-injecting an EVENTINJ NMI into L2 on
>>> the behalf of L1.
>>> That's when "nmi_l1_to_l2" is true.
>>   hmm,. trying to understand the event re-injection flow -
>> First L1 (non-vnmi) injecting event to L2 guest, in-turn
>> intercepted by L0, 
> 
> That's right, the L1's VMRUN of L2 gets intercepted by L0.
> 
>> L0 sees event injection through EVTINJ
> 
> It sees that L1 wants to inject an NMI into L2 via VMCB12 EVTINJ.
> 
>> so sets the 'nmi_l1_to_l2' var 
> 
> That's right, L0 needs to keep track of this fact.
> 
>> and then L0 calls svm_inject_nmi()
> 
> Not yet - at this point svm_inject_nmi() is NOT called
> (rather than, VMCB12 EVTINJ is directly copied into VMCB02 EVTINJ).
> 
> Now L0 does the actual VMRUN of L2.
> 
> Let's say that there is an intervening VMExit during delivery of
> that NMI to L2, of type which is handled by L0 (perhaps a NPF on
> L2 IDT or so).
> 
> In this case the NMI will be returned in VMCB02 EXITINTINFO and
> needs to be re-injected into L2 on the next VMRUN,
> again via EVTINJ.
> 
> That's when svm_inject_nmi() will get called to re-inject
> that NMI.
> 
>> to re-inject event in L2 - is that correct (nmi_l1_to_l2) flow?
> Hope the flow is clear now.
> 

Yes, Thank-you for the clarification :).

Santosh.

>>
>> Thanks,.
>> Santosh
> 
> Thanks,
> Maciej