linux-kernel - Re: [PATCH 3/3] KVM: nVMX: Clear IDT vectoring on nested VM-Exit for double/triple fault

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Date:   Thu, 7 Apr 2022 16:42:06 +0800
From:   Xiaoyao Li <xiaoyao.li@...el.com>
To:     Sean Christopherson <seanjc@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>
Cc:     Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, Chenyi Qiang <chenyi.qiang@...el.com>
Subject: Re: [PATCH 3/3] KVM: nVMX: Clear IDT vectoring on nested VM-Exit for
 double/triple fault

On 4/7/2022 4:02 PM, Xiaoyao Li wrote:
> On 4/7/2022 8:23 AM, Sean Christopherson wrote:
>> Clear the IDT vectoring field in vmcs12 on next VM-Exit due to a double
>> or triple fault.  Per the SDM, a VM-Exit isn't considered to occur during
>> event delivery if the exit is due to an intercepted double fault or a
>> triple fault.  Opportunistically move the default clearing (no event
>> "pending") into the helper so that it's more obvious that KVM does indeed
>> handle this case.
>>
>> Note, the double fault case is worded rather wierdly in the SDM:
>>
>>    The original event results in a double-fault exception that causes the
>>    VM exit directly.
>>
>> Temporarily ignoring injected events, double faults can _only_ occur if
>> an exception occurs while attempting to deliver a different exception,
>> i.e. there's _always_ an original event.  And for injected double fault,
>> while there's no original event, injected events are never subject to
>> interception.
>>
>> Presumably the SDM is calling out that a the vectoring info will be valid
>> if a different exit occurs after a double fault, e.g. if a #PF occurs and
>> is intercepted while vectoring #DF, then the vectoring info will show the
>> double fault. 
> 
> Wouldn't it be a tripe fault exit in this case?

It won't since #PF is intercepted by exception bitmap in your case.

>> In other words, the clause can simply be read as:
>>
>>    The VM exit is caused by a double-fault exception.
>