On 12/02/2016 17:21, Suravee Suthikulpanit wrote: > Hi Paolo, > > On 02/12/2016 10:55 PM, Paolo Bonzini wrote: >>> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c >>> >index 4244c2b..2def290 100644 >>> >--- a/arch/x86/kvm/x86.c >>> >+++ b/arch/x86/kvm/x86.c >>> >@@ -8087,7 +8087,9 @@ int kvm_arch_vcpu_runnable(struct kvm_vcpu *vcpu) >>> > if (is_guest_mode(vcpu) && kvm_x86_ops->check_nested_events) >>> > kvm_x86_ops->check_nested_events(vcpu, false); >>> > >>> >- return kvm_vcpu_running(vcpu) || kvm_vcpu_has_events(vcpu); >>> >+ return (kvm_vcpu_running(vcpu) || kvm_vcpu_has_events(vcpu) || >>> >+ (kvm_x86_ops->apicv_intr_pending && >>> >+ kvm_x86_ops->apicv_intr_pending(vcpu))); >>> > } >> I think this is not necessary. What you need is to make kvm_lapic's >> regs field point to the backing page. Then when the processor writes to >> IRR, kvm_apic_has_interrupt (called through kvm_vcpu_has_events) will >> see it. >> >> avic_pending_cnt shouldn't be necessary either. >> >> Paolo > > So, the other thing I am using the avic_pending_cnt for is for the part > 2 of the series (to enable AVIC support in IOMMU) that I am planning to > send out later. However, it might be good to discuss this at this point. It's better to discuss it later. For now, I would prefer the AVIC patches to be as clean as possible, and not know about the IOMMU at all. Also, there are a lot of assumptions about how to use kvm_lapic's regs field for APIC virtualization---dating back to when Intel only virtualized the TPR field. Deviating for that would be a recipe for trouble. :) Regarding the IOMMU, I'm actually very happy with the way the Intel VT-d posted interrupts patches worked out, so I would be even more happy if everything you do fits in the same scheme and reuses the same hooks! :D > When the IOMMU cannot inject interrupts into the guest vcpu due to it is > not running (therefore, it cannot doorbell the vcpu directly), it logs > the interrupt in the GA log buffer. Where is this documented? > Then it generates interrupt to > notify the IOMMU driver that it needs to handle the log entry. Here, the > IOMMU driver will end up notifying the SVM to scheduling the VCPU in to > process interrupt. > > Here, I have run into issue where the vcpu often goes into idle (i.e. > scheduled out), and ended up causing IOMMU to generate a lot of the > entries in the GA log. This really hurts device pass-through performance > (e.g. for XGBE NIC). > > So, what I ended up experimenting with is to set the avic_pending_cnt to > a larger value (i.e. avic_ga_log_threshold) whenever we processing the > GA log entry. The intention is to delay the vcpu schedule out in > expecting that there might be more interrupts coming in soon. I also > make this threshold value tunable as a module_param. > > This actually works well in my experiment, where I can actually get > about 5% speed up in my netperf test on XGBE NIC pass-through test. > However, I am not sure if this is an acceptable approach. Actually, I > think it's similar to the halt_poll_ns, but specifically for IOMMU GA > log in this case. Have you retested now that the halt_poll_ns mechanism is dynamic and enabled by default? If I read patch 9 right, halt_poll_ns would delay vcpu_put and IsRunning=0. Hopefully this is enough to avoid this kind of notification and make the issue moot. Paolo -- 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
