linux-kernel - Re: [PATCH v3] KVM: VMX: Enable Notify VM exit

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Date:   Sat, 26 Feb 2022 12:07:25 +0800
From:   Xiaoyao Li <xiaoyao.li@...el.com>
To:     Paolo Bonzini <pbonzini@...hat.com>,
        Jim Mattson <jmattson@...gle.com>,
        Chenyi Qiang <chenyi.qiang@...el.com>
Cc:     Sean Christopherson <seanjc@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] KVM: VMX: Enable Notify VM exit

On 2/25/2022 11:13 PM, Paolo Bonzini wrote:
> On 2/25/22 16:12, Xiaoyao Li wrote:
>>>>>
>>>>
>>>> I don't like the idea of making things up without notifying userspace
>>>> that this is fictional. How is my customer running nested VMs supposed
>>>> to know that L2 didn't actually shutdown, but L0 killed it because the
>>>> notify window was exceeded? If this information isn't reported to
>>>> userspace, I have no way of getting the information to the customer.
>>>
>>> Then, maybe a dedicated software define VM exit for it instead of 
>>> reusing triple fault?
>>>
>>
>> Second thought, we can even just return Notify VM exit to L1 to tell 
>> L2 causes Notify VM exit, even thought Notify VM exit is not exposed 
>> to L1.
> 
> That might cause NULL pointer dereferences or other nasty occurrences.

IMO, a well written VMM (in L1) should handle it correctly.

L0 KVM reports no Notify VM Exit support to L1, so L1 runs without 
setting Notify VM exit. If a L2 causes notify_vm_exit with 
invalid_vm_context, L0 just reflects it to L1. In L1's view, there is no 
support of Notify VM Exit from VMX MSR capability. Following L1 handler 
is possible:

a)	if (notify_vm_exit available & notify_vm_exit enabled) {
		handle in b)	
	} else {
		report unexpected vm exit reason to userspace;
	}

b) 	similar handler like we implement in KVM:
	if (!vm_context_invalid)
		re-enter guest;
	else
		report to userspace;

c)	no Notify VM Exit related code (e.g. old KVM), it's treated as 
unsupported exit reason

As long as it belongs to any case above, I think L1 can handle it 
correctly. Any nasty occurrence should be caused by incorrect handler in 
L1 VMM, in my opinion.

> Paolo
>