Re: [PART1 RFC 6/9] svm: Add interrupt injection via AVIC

[Paolo Bonzini <pbonzini@redhat.com>]

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