linux-kernel - Re: [PATCH v2 1/4] KVM: nSVM: cancel KVM_REQ_GET_NESTED_STATE

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Date:   Thu, 7 Jan 2021 18:59:59 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>,
        Maxim Levitsky <mlevitsk@...hat.com>
Cc:     kvm@...r.kernel.org, Vitaly Kuznetsov <vkuznets@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Borislav Petkov <bp@...en8.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel@...r.kernel.org,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        Ingo Molnar <mingo@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Joerg Roedel <joro@...tes.org>,
        Jim Mattson <jmattson@...gle.com>
Subject: Re: [PATCH v2 1/4] KVM: nSVM: cancel KVM_REQ_GET_NESTED_STATE_PAGES
 on nested vmexit

On 07/01/21 18:51, Paolo Bonzini wrote:
> On 07/01/21 18:00, Sean Christopherson wrote:
>> Ugh, I assume this is due to one of the "premature" 
>> nested_ops->check_events()
>> calls that are necessitated by the event mess?  I'm guessing 
>> kvm_vcpu_running()
>> is the culprit?
>>
>> If my assumption is correct, this bug affects nVMX as well.
> 
> Yes, though it may be latent.  For SVM it was until we started 
> allocating svm->nested on demand.
> 
>> Rather than clear the request blindly on any nested VM-Exit, what
>> about something like the following?
> 
> I think your patch is overkill, KVM_REQ_GET_NESTED_STATE_PAGES is only 
> set from KVM_SET_NESTED_STATE so it cannot happen while the VM runs.

... and when leaving SMM.  But in either case, there cannot be something 
else causing a nested vmexit before the request is set, because SMM does 
not support VMX operation.  So I still don't think that it justifies the 
extra code and indirection.

Paolo