* Long interrupt latency in guest
@ 2017-01-25 23:57 Su, David W
2017-01-30 18:03 ` Radim Krčmář
0 siblings, 1 reply; 2+ messages in thread
From: Su, David W @ 2017-01-25 23:57 UTC (permalink / raw)
To: kvm
We use vfio-pci to expose a NIC device to a guest and a packet generator generating time stamped packets to measure latency. The NIC is programmed to generate an interrupt when it receives a packet. qemu cpu thread is bound to a fixed CPU core and the NIC interrupts are bound to the same core. The host CPU supports APIC virtualization. We have observed sometimes an interrupt is delayed for a relatively long time (mini seconds) before being delivered to the guest.
It seems there is a small window in the function vcpu_enter_guest in arch/x86/kvm/x86.c, where an interrupt from a device managed by vfio-pci is queued in PIR after PIR has synchronized to VIRR. This will cause the interrupt not delivered to the guest until the next VM exit-entry cycle. Refer to the code snippet below, if a device interrupt arrives after the KVM_REQ_EVENT check block and before local_irq_disable(), the interrupt request will be in PIR but not in VIRR. In a worst case scenario, the interrupt would get lost if another interrupt from the device arrived before a VM exit occurred.
/*
* KVM_REQ_EVENT is not set when posted interrupts are set by
* VT-d hardware, so we have to update RVI unconditionally.
*/
if (kvm_lapic_enabled(vcpu)) {
/*
* Update architecture specific hints for APIC
* virtual interrupt delivery.
*/
if (kvm_x86_ops->hwapic_irr_update)
kvm_x86_ops->hwapic_irr_update(vcpu,
kvm_lapic_find_highest_irr(vcpu));
}
if (kvm_check_request(KVM_REQ_EVENT, vcpu) || req_int_win) {
...
...
...
local_irq_disable();
if (vcpu->mode == EXITING_GUEST_MODE || vcpu->requests
|| need_resched() || signal_pending(current)) {
Moving the "if (kvm_lapic_enabled(vcpu))" block to after the "if (vcpu->mode == EXITING_GUEST_MODE ..." block resolved the long interrupt latency issue in my limited testing, but I'm not sure if this is going to break something else.
^ permalink raw reply [flat|nested] 2+ messages in thread
* Re: Long interrupt latency in guest
2017-01-25 23:57 Long interrupt latency in guest Su, David W
@ 2017-01-30 18:03 ` Radim Krčmář
0 siblings, 0 replies; 2+ messages in thread
From: Radim Krčmář @ 2017-01-30 18:03 UTC (permalink / raw)
To: Su, David W; +Cc: kvm
2017-01-25 23:57+0000, Su, David W:
> We use vfio-pci to expose a NIC device to a guest and a packet generator generating time stamped packets to measure latency. The NIC is programmed to generate an interrupt when it receives a packet. qemu cpu thread is bound to a fixed CPU core and the NIC interrupts are bound to the same core. The host CPU supports APIC virtualization. We have observed sometimes an interrupt is delayed for a relatively long time (mini seconds) before being delivered to the guest.
>
> It seems there is a small window in the function vcpu_enter_guest in arch/x86/kvm/x86.c, where an interrupt from a device managed by vfio-pci is queued in PIR after PIR has synchronized to VIRR. This will cause the interrupt not delivered to the guest until the next VM exit-entry cycle. Refer to the code snippet below, if a device interrupt arrives after the KVM_REQ_EVENT check block and before local_irq_disable(), the interrupt request will be in PIR but not in VIRR. In a worst case scenario, the interrupt would get lost if another interrupt from the device arrived before a VM exit occurred.
Good analysis.
> /*
> * KVM_REQ_EVENT is not set when posted interrupts are set by
> * VT-d hardware, so we have to update RVI unconditionally.
> */
> if (kvm_lapic_enabled(vcpu)) {
> /*
> * Update architecture specific hints for APIC
> * virtual interrupt delivery.
> */
> if (kvm_x86_ops->hwapic_irr_update)
> kvm_x86_ops->hwapic_irr_update(vcpu,
> kvm_lapic_find_highest_irr(vcpu));
> }
>
> if (kvm_check_request(KVM_REQ_EVENT, vcpu) || req_int_win) {
>
> ...
> ...
> ...
>
> local_irq_disable();
>
> if (vcpu->mode == EXITING_GUEST_MODE || vcpu->requests
> || need_resched() || signal_pending(current)) {
>
>
> Moving the "if (kvm_lapic_enabled(vcpu))" block to after the "if (vcpu->mode == EXITING_GUEST_MODE ..." block resolved the long interrupt latency issue in my limited testing, but I'm not sure if this is going to break something else.
hwapic_irr_update() or kvm_lapic_find_highest_irr() might set
vcpu->requests, so they should be just after local_irq_disable().
A drawback is that kvm_lapic_find_highest_irr() sets KVM_REQ_EVENT on
every PIR sync and thus forces useless re-runs of vcpu_enter_guest().
Paolo's pending patches eliminated the useless KVM_REQ_EVENT and also
moved the update just after local_irq_disable().
(Without noticing that it fixes a bug.)
Please verify that the delay goes away with patches from
https://lkml.org/lkml/2016/12/19/352
Thanks.
^ permalink raw reply [flat|nested] 2+ messages in thread
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