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@xxxxxxxxxxxxxxx/msg54478.html > >>>>>>>>>> Another description might also come in handy: > >>>>>>>>>> http://www.mail-archive.com/kvm@xxxxxxxxxxxxxxx/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(). -- Gleb. -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
