linux-kernel - Re: [PATCH v2] KVM: VMX: Enable Notify VM exit

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Date:   Tue, 7 Sep 2021 21:45:12 +0800
From:   Xiaoyao Li <xiaoyao.li@...el.com>
To:     Sean Christopherson <seanjc@...gle.com>,
        Chenyi Qiang <chenyi.qiang@...el.com>
Cc:     pbonzini@...hat.com, vkuznets@...hat.com, wanpengli@...cent.com,
        jmattson@...gle.com, joro@...tes.org, tglx@...utronix.de,
        mingo@...hat.com, bp@...en8.de, hpa@...or.com, x86@...nel.org,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] KVM: VMX: Enable Notify VM exit

On 9/3/2021 12:36 AM, Sean Christopherson wrote:
> On Thu, Sep 02, 2021, Sean Christopherson wrote:
>> On Tue, Aug 03, 2021, Xiaoyao Li wrote:
>>> On 8/2/2021 11:46 PM, Sean Christopherson wrote:
>>>>>>> @@ -5642,6 +5653,31 @@ static int handle_bus_lock_vmexit(struct kvm_vcpu *vcpu)
>>>>>>>     	return 0;
>>>>>>>     }
>>>>>>> +static int handle_notify(struct kvm_vcpu *vcpu)
>>>>>>> +{
>>>>>>> +	unsigned long exit_qual = vmx_get_exit_qual(vcpu);
>>>>>>> +
>>>>>>> +	if (!(exit_qual & NOTIFY_VM_CONTEXT_INVALID)) {
>>>>>>
>>>>>> What does CONTEXT_INVALID mean?  The ISE doesn't provide any information whatsoever.
>>>>>
>>>>> It means whether the VM context is corrupted and not valid in the VMCS.
>>>>
>>>> Well that's a bit terrifying.  Under what conditions can the VM context become
>>>> corrupted?  E.g. if the context can be corrupted by an inopportune NOTIFY exit,
>>>> then KVM needs to be ultra conservative as a false positive could be fatal to a
>>>> guest.
>>>>
>>>
>>> Short answer is no case will set the VM_CONTEXT_INVALID bit.
>>
>> But something must set it, otherwise it wouldn't exist.  

For existing Intel silicon, no case will set it. Maybe in the future new 
case will set it.

> The condition(s) under
>> which it can be set matters because it affects how KVM should respond.  E.g. if
>> the guest can trigger VM_CONTEXT_INVALID at will, then we should probably treat
>> it as a shutdown and reset the VMCS.
> 
> Oh, and "shutdown" would be relative to the VMCS, i.e. if L2 triggers a NOTIFY
> exit with VM_CONTEXT_INVALID then KVM shouldn't kill the entire VM.  The least
> awful option would probably be to synthesize a shutdown VM-Exit to L1.  That
> won't communicate to L1 that vmcs12 state is stale/bogus, but I don't see any way
> to handle that via an existing VM-Exit reason :-/
> 
>> But if VM_CONTEXT_INVALID can occur if and only if there's a hardware/ucode
>> issue, then we can do:
>>
>> 	if (KVM_BUG_ON(exit_qual & NOTIFY_VM_CONTEXT_INVALID, vcpu->kvm))
>> 		return -EIO;
>>
>> Either way, to enable this by default we need some form of documentation that
>> describes what conditions lead to VM_CONTEXT_INVALID.

I still don't know why the conditions lead to it matters. I think the 
consensus is that once VM_CONTEXT_INVALID happens, the vcpu can no 
longer run. Either KVM_BUG_ON() or a specific EXIT to userspace should 
be OK?