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

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

On 2013-02-21 14:13, Gleb Natapov wrote:
> On Thu, Feb 21, 2013 at 11:33:30AM +0100, Jan Kiszka wrote:
>> On 2013-02-21 11:28, Jan Kiszka wrote:
>>> 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.
>>
>> Explained in 51cfe38ea5: idt_vectoring_info_field and vm_exit_intr_info
>> must not be valid at the same time.
>>
> Interestingly, if we transfer interrupt from idt_vectoring_info into
> arch VCPU state we can drop this check because vmx_interrupt_allowed()
> will not be called while there is an event to reinject. 51cfe38ea5 still
> does not explain why nested_run_pending is needed. We cannot #vmexit
> without entering L2, but we can undo VMLAUNCH/VMRESUME emulation leaving
> rip pointing to the instruction. We can start by moving
> skip_emulated_instruction() from nested_vmx_run() to nested_vmx_vmexit().

That generally does not help to inject/report an external IRQ to L1 as
L1 runs with IRQs disabled around VMLAUNCH/RESUME. Thus, the only way to
report this IRQ is a VMEXIT. I think the ordering is hard: first inject
what L1 wants to send to L2, then VMEXIT with that external IRQ in
VM_EXIT_INTR_INFO.

Jan

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