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

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Message-ID: <3752b74b-74e1-00fd-d80d-41104e07fe95@maciej.szmigiero.name>
Date:   Thu, 25 Aug 2022 14:45:53 +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 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.

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

>
>	if (is_guest_mode(&svm->vcpu))
>		return svm->nested.vmcb02.ptr;
		^ this should be always non-NULL.

So get_vnmi_vmcb() will return VMCB02 pointer in our case, not NULL...

> 
> Thanks,
> Santosh
> 
>> This field in VMCB02 comes from nested_vmcb02_prepare_control() which
>> in the !nested_vnmi_enabled() case (L1 is not using vNMI) copies these bits
>> from VMCB01:
>>> int_ctl_vmcb01_bits |= (V_NMI_PENDING | V_NMI_ENABLE | V_NMI_MASK);
>>
>> So in this case (L0 uses vNMI) V_NMI_ENABLE will be set in VMCB01, right?
>>
>> This bit will then be copied to VMCB02 

... and due to the above is_vnmi_enabled() will return true, so
re-injection will attempt to use vNMI instead of EVTINJ (wrong).

>>> 2) L0 & L1 both vnmi disabled.
>>
>> This case is ok.
>>
>>>
>>> In both cases the vnmi check will fail for L1 and execution path
>>> will fall back to default - right?
>>>
>>> Thanks,
>>> Santosh
>>

Thanks,
Maciej