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

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Message-ID: <f96b867f-4c32-4950-8508-234fe2cda7b9@maciej.szmigiero.name>
Date:   Wed, 24 Aug 2022 14:56:57 +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 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.

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 so re-injection will attempt to use
vNMI instead of EVTINJ.

> 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