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

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

On 2013-02-21 11:18, Jan Kiszka wrote:
> On 2013-02-21 11:06, Gleb Natapov wrote:
>> On Thu, Feb 21, 2013 at 10:43:57AM +0100, Jan Kiszka wrote:
>>> On 2013-02-21 10:22, Gleb Natapov wrote:
>>>> On Wed, Feb 20, 2013 at 06:50:50PM +0100, Jan Kiszka wrote:
>>>>> 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.
>>>>>
>>>> Why would we trigger enable_irq_window()? enable_irq_window() triggers
>>>> only if interrupt is pending in one of irq chips, not in architectural
>>>> VCPU state.
>>>
>>> Precisely this is the case if an IRQ for L1 arrived while we tried to
>>> enter L2 and caused the cancellation above.
>>>
>> But during next entry the cancelled interrupt is transfered
>> from architectural VCPU state to VM_ENTRY_INTR_INFO_FIELD by
>> inject_pending_event()->vmx_inject_irq(), so at the point where
>> enable_irq_window() is called the state is exactly the same no matter
>> whether we canceled interrupt or not during previous entry attempt. What
>> am I missing?
> 
> Maybe that we normally either have an external IRQ pending in some IRQ
> chip or in the VCPU architectural state, not both at the same time? By
> transferring something that doesn't come from a virtual IRQ chip of L0
> (but from the one in L1) into the architectural state, we break this
> assumption.
> 
>> Oh may be I am missing that if we do not cancel interrupt
>> then inject_pending_event() will skip
>>   if (vcpu->arch.interrupt.pending)
>>     ....
> 
> If we do not cancel, we will not inject at all (due to missing
> KVM_REQ_EVENT).
> 
>> and will inject interrupt from APIC that caused cancellation of previous
>> entry, but then this is a bug since this new interrupt will overwrite
>> the one that is still in VM_ENTRY_INTR_INFO_FIELD from previous entry
>> attempt and there may be another pending interrupt in APIC anyway that
>> will cause enable_irq_window() too.
> 
> Maybe the issue is that we do not properly simulate a VMEXIT on an
> external interrupt during vmrun (like SVM does). Need to check for this
> case again...

static int vmx_interrupt_allowed(struct kvm_vcpu *vcpu)
{
	if (is_guest_mode(vcpu) && nested_exit_on_intr(vcpu)) {
		struct vmcs12 *vmcs12 = get_vmcs12(vcpu);
		if (to_vmx(vcpu)->nested.nested_run_pending ||
		    (vmcs12->idt_vectoring_info_field &
		     VECTORING_INFO_VALID_MASK))
			return 0;
		nested_vmx_vmexit(vcpu);
		vmcs12->vm_exit_reason = EXIT_REASON_EXTERNAL_INTERRUPT;
		vmcs12->vm_exit_intr_info = 0;
		...

I do not understand ATM why we refuse to simulate a vmexit due to an
external interrupt when we are about to run L2 or have something in
idt_vectoring_info_field. The external interrupt would not overwrite
idt_vectoring_info_field but should end up in vm_exit_intr_info.

Jan

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