Re: [PATCH v3 15/16] kvm: x86: ioapic: Lazy update IOAPIC EOI

From: Paolo Bonzini
Date: Wed Oct 09 2019 - 05:21:51 EST

On 13/09/19 21:01, Suthikulpanit, Suravee wrote:
> /*
> + * In case APICv accelerate EOI write and do not trap,
> + * in-kernel IOAPIC will not be able to receive the EOI.
> + * In this case, we do lazy update of the pending EOI when
> + * trying to set IOAPIC irq.
> + */
> + if (kvm_apicv_eoi_accelerate(ioapic->kvm, edge))
> + ioapic_lazy_update_eoi(ioapic, irq);
> +

This is okay for the RTC, and in fact I suggest that you make it work
like this even for Intel. This will get rid of kvm_apicv_eoi_accelerate
and be nicer overall.

However, it cannot work for the in-kernel PIT, because it is currently
checking ps->irq_ack before kvm_set_irq. Unfortunately, the in-kernel
PIT is relying on the ack notifier to timely queue the pt->worker work
item and reinject the missed tick.

Thus, you cannot enable APICv if ps->reinject is true.

Perhaps you can make kvm->arch.apicv_state a disabled counter? Then
Hyper-V can increment it when enabled, PIT can increment it when
ps->reinject becomes true and decrement it when it becomes false;
finally, svm.c can increment it when an SVM guest is launched and
increment/decrement it around ExtINT handling?

(This conflicts with some of the suggestions I made earlier, which
implied the existence of apicv_state, but everything should if anything
become easier).