[PATCH v2 11/43] KVM: Don't block+unblock when halt-polling is successful

[Maxim Levitsky]

On Thu, 2021-12-02 at 12:20 +0200, Maxim Levitsky wrote:
> On Mon, 2021-11-29 at 17:25 +0000, Sean Christopherson wrote:
> > On Mon, Nov 29, 2021, Maxim Levitsky wrote:
> > > (This thing is that when you tell the IOMMU that a vCPU is not running,
> > > Another thing I discovered that this patch series totally breaks my VMs,
> > > without cpu_pm=on The whole series (I didn't yet bisect it) makes even my
> > > fedora32 VM be very laggy, almost unusable, and it only has one
> > > passed-through device, a nic).
> > 
> > Grrrr, the complete lack of comments in the KVM code and the separate paths for
> > VMX vs SVM when handling HLT with APICv make this all way for difficult to
> > understand than it should be.
> > 
> > The hangs are likely due to:
> > 
> >   KVM: SVM: Unconditionally mark AVIC as running on vCPU load (with APICv)
> > 
> > If a posted interrupt arrives after KVM has done its final search through the vIRR,
> > but before avic_update_iommu_vcpu_affinity() is called, the posted interrupt will
> > be set in the vIRR without triggering a host IRQ to wake the vCPU via the GA log.
> > 
> > I.e. KVM is missing an equivalent to VMX's posted interrupt check for an outstanding
> > notification after switching to the wakeup vector.
> > 
> > For now, the least awful approach is sadly to keep the vcpu_(un)blocking() hooks.
> > Unlike VMX's PI support, there's no fast check for an interrupt being posted (KVM
> > would have to rewalk the vIRR), no easy to signal the current CPU to do wakeup (I
> > don't think KVM even has access to the IRQ used by the owning IOMMU), and there's
> > no simplification of load/put code.
> 
> I have an idea.
>  
> Why do we even use/need the GA log?
> Why not, just disable the 'guest mode' in the iommu and let it sent good old normal interrupt
> when a vCPU is not running, just like we do when we inhibit the AVIC?
>  
> GA log makes all devices that share an iommu (there are 4 iommus per package these days,
> some without useful devices) go through a single (!) msi like interrupt,
> which is even for some reason implemented by a threaded IRQ in the linux kernel.


Yep, this gross hack works!


diff --git a/arch/x86/kvm/svm/avic.c b/arch/x86/kvm/svm/avic.c
index 958966276d00b8..6136b94f6b5f5e 100644
--- a/arch/x86/kvm/svm/avic.c
+++ b/arch/x86/kvm/svm/avic.c
@@ -987,8 +987,9 @@ void avic_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
                entry |= AVIC_PHYSICAL_ID_ENTRY_IS_RUNNING_MASK;
 
        WRITE_ONCE(*(svm->avic_physical_id_cache), entry);
-       avic_update_iommu_vcpu_affinity(vcpu, h_physical_id,
-                                       svm->avic_is_running);
+
+       svm_set_pi_irte_mode(vcpu, svm->avic_is_running);
+       avic_update_iommu_vcpu_affinity(vcpu, h_physical_id, true);
 }
 
 void avic_vcpu_put(struct kvm_vcpu *vcpu)
@@ -997,8 +998,9 @@ void avic_vcpu_put(struct kvm_vcpu *vcpu)
        struct vcpu_svm *svm = to_svm(vcpu);
 
        entry = READ_ONCE(*(svm->avic_physical_id_cache));
-       if (entry & AVIC_PHYSICAL_ID_ENTRY_IS_RUNNING_MASK)
-               avic_update_iommu_vcpu_affinity(vcpu, -1, 0);
+       if (entry & AVIC_PHYSICAL_ID_ENTRY_IS_RUNNING_MASK) {
+               svm_set_pi_irte_mode(vcpu, false);
+       }
 
        entry &= ~AVIC_PHYSICAL_ID_ENTRY_IS_RUNNING_MASK;
        WRITE_ONCE(*(svm->avic_physical_id_cache), entry);
> 


GA log interrupts almost gone (there are still few because svm_set_pi_irte_mode sets is_running false)
devices works as expected sending normal interrupts unless guest is loaded, then normal interrupts disappear,
as expected.

Best regards,
	Maxim Levitsky

>  
> Best regards,
> 	Maxim Levitsky
> 
> > If the scheduler were changed to support waking in the sched_out path, then I'd be
> > more inclined to handle this in avic_vcpu_put() by rewalking the vIRR one final
> > time, but for now it's not worth it.
> > 
> > > If I apply though only the patch series up to this patch, my fedora VM seems
> > > to work fine, but my windows VM still locks up hard when I run 'LatencyTop'
> > > in it, which doesn't happen without this patch.
> > 
> > Buy "run 'LatencyTop' in it", do you mean running something in the Windows guest?
> > The only search results I can find for LatencyTop are Linux specific.
> > 
> > > So far the symptoms I see is that on VCPU 0, ISR has quite high interrupt
> > > (0xe1 last time I seen it), TPR and PPR are 0xe0 (although I have seen TPR to
> > > have different values), and IRR has plenty of interrupts with lower priority.
> > > The VM seems to be stuck in this case. As if its EOI got lost or something is
> > > preventing the IRQ handler from issuing EOI.
> > >  
> > > LatencyTop does install some form of a kernel driver which likely does meddle
> > > with interrupts (maybe it sends lots of self IPIs?).
> > >  
> > > 100% reproducible as soon as I start monitoring with LatencyTop.
> > >  
> > > Without this patch it works (or if disabling halt polling),
> > 
> > Huh.  I assume everything works if you disable halt polling _without_ this patch
> > applied?
> > 
> > If so, that implies that successful halt polling without mucking with vCPU IOMMU
> > affinity is somehow problematic.  I can't think of any relevant side effects other
> > than timing.
> >