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

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Message-ID: <ca464c8c-872f-f86b-6fca-7ef7b374a304@amd.com>
Date:   Fri, 26 Aug 2022 15:05:46 +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 8/25/2022 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, L0 sees event injection through EVTINJ
so sets the 'nmi_l1_to_l2' var and then L0 calls svm_inject_nmi()
to re-inject event in L2 - is that correct (nmi_l1_to_l2) flow?

Thanks,.
Santosh
> Since the code is physically running on L0 (which makes use of vNMI)
> it has the "vnmi" param set.
> 
> So is_vnmi_enabled() will return true and vNMI will be used
> for the re-injection instead of the required EVENTINJ.
> 
>> In L0 case (vnmi enabled) - vnmi param will be true.
>>
>> So in L1 case, is_vnmi_enabled() will return false and
>> in L0 case will return true.
>>
>> Thanks,
>> Santosh
> 
> Thanks,
> Maciej