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

[Maxim Levitsky]

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.

 
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.
>