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

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Date:   Thu, 25 Aug 2022 16:16:10 +0200
From:   "Maciej S. Szmigiero" <mail@...iej.szmigiero.name>
To:     "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 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.

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.

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