Re: [PATCH] KVM: nVMX: Rework event injection and recovery

[Jan Kiszka <jan.kiszka@siemens.com>]

On 2013-02-20 18:24, Jan Kiszka wrote:
> On 2013-02-20 18:01, Gleb Natapov wrote:
>> On Wed, Feb 20, 2013 at 03:37:51PM +0100, Jan Kiszka wrote:
>>> On 2013-02-20 15:14, Nadav Har'El wrote:
>>>> Hi,
>>>>
>>>> By the way, if you haven't seen my description of why the current code
>>>> did what it did, take a look at
>>>> http://www.mail-archive.com/kvm@vger.kernel.org/msg54478.html
>>>> Another description might also come in handy:
>>>> http://www.mail-archive.com/kvm@vger.kernel.org/msg54476.html
>>>>
>>>> On Wed, Feb 20, 2013, Jan Kiszka wrote about "[PATCH] KVM: nVMX: Rework event injection and recovery":
>>>>> This aligns VMX more with SVM regarding event injection and recovery for
>>>>> nested guests. The changes allow to inject interrupts directly from L0
>>>>> to L2.
>>>>>
>>>>> One difference to SVM is that we always transfer the pending event
>>>>> injection into the architectural state of the VCPU and then drop it from
>>>>> there if it turns out that we left L2 to enter L1.
>>>>
>>>> Last time I checked, if I'm remembering correctly, the nested SVM code did
>>>> something a bit different: After the exit from L2 to L1 and unnecessarily
>>>> queuing the pending interrupt for injection, it skipped one entry into L1,
>>>> and as usual after the entry the interrupt queue is cleared so next time
>>>> around, when L1 one is really entered, the wrong injection is not attempted.
>>>>
>>>>> VMX and SVM are now identical in how they recover event injections from
>>>>> unperformed vmlaunch/vmresume: We detect that VM_ENTRY_INTR_INFO_FIELD
>>>>> still contains a valid event and, if yes, transfer the content into L1's
>>>>> idt_vectoring_info_field.
>>>>
>>>>> To avoid that we incorrectly leak an event into the architectural VCPU
>>>>> state that L1 wants to inject, we skip cancellation on nested run.
>>>>
>>>> I didn't understand this last point.
>>>
>>> - prepare_vmcs02 sets event to be injected into L2
>>> - while trying to enter L2, a cancel condition is met
>>> - we call vmx_cancel_interrupts but should now avoid filling L1's event
>>>   into the arch event queues - it's kept in vmcs12
>>>
>> But what if we put it in arch event queue? It will be reinjected during
>> next entry attempt, so nothing bad happens and we have one less if() to explain,
>> or do I miss something terrible that will happen?
> 
> I started without that if but ran into troubles with KVM-on-KVM (L1
> locks up). Let me dig out the instrumentation and check the event flow
> again.

OK, got it again: If we transfer an IRQ that L1 wants to send to L2 into
the architectural VCPU state, we will also trigger enable_irq_window.
And that raises KVM_REQ_IMMEDIATE_EXIT again as it thinks L0 wants
inject. That will send us into an endless loop.

Not sure if we can and should handle this scenario in enable_irq_window
in a nicer way. Open for suggestions.

Jan

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