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? Oh may be I am missing that if we do not cancel interrupt then inject_pending_event() will skip if (vcpu->arch.interrupt.pending) .... 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. -- 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
