Paolo Bonzini <pbonzini@xxxxxxxxxx> writes: > On 28/03/19 21:31, Vitaly Kuznetsov wrote: >> >> The 'hang' scenario develops like this: >> 1) Hyper-V boots and QEMU is trying to inject two irq simultaneously. One >> of them is level-triggered. KVM injects the edge-triggered one and >> requests immediate exit to inject the level-triggered: >> >> kvm_set_irq: gsi 23 level 1 source 0 >> kvm_msi_set_irq: dst 0 vec 80 (Fixed|physical|level) >> kvm_apic_accept_irq: apicid 0 vec 80 (Fixed|edge) >> kvm_msi_set_irq: dst 0 vec 96 (Fixed|physical|edge) >> kvm_apic_accept_irq: apicid 0 vec 96 (Fixed|edge) >> kvm_nested_vmexit_inject: reason EXTERNAL_INTERRUPT info1 0 info2 0 int_info 80000060 int_info_err 0 >> >> 2) Hyper-V requires one of its VMs to run to handle the situation but >> immediate exit happens: >> >> kvm_entry: vcpu 0 >> kvm_exit: reason VMRESUME rip 0xfffff80006a40115 info 0 0 >> kvm_entry: vcpu 0 >> kvm_exit: reason PREEMPTION_TIMER rip 0xfffff8022f3d8350 info 0 0 >> kvm_nested_vmexit: rip fffff8022f3d8350 reason PREEMPTION_TIMER info1 0 info2 0 int_info 0 int_info_err 0 >> kvm_nested_vmexit_inject: reason EXTERNAL_INTERRUPT info1 0 info2 0 int_info 80000050 int_info_err 0 > > I supposed before this there was an eoi for vector 96? AFAIR: no, it seems that it is actually the VM it is trying to resume (Windows partition?) which needs to do some work and with the preemtion timer of 0 we don't allow it to. > > The main issue with your patch is that the preemption timer is buggy on > some processors (it runs too fast) and on those processors we shouldn't > use it with nonzero deadline. In particular because it runs too fast it > may not hide the bug. > > I think level-triggered interrupts are required for the bug to show. > Edge-triggered interrupts usually have to be acknowledged with a device > register before the host device will trigger another interrupt; or at > least the interrupt event, for example an incoming network packet, must > happen again. This way, when the guest hangs it puts some back pressure > on the host. > > I think we should do in QEMU the same fix that was done in the in-kernel > IOAPIC. Yes, I have this in my plan. Stay tuned! -- Vitaly
