Re: [patch 1/2] KVM: hlt emulation should take in-kernel APIC/PIT timers into account

Avi Kivity
2008-04-11T12:12:41Z

Marcelo Tosatti wrote:
> Timers that fire between guest hlt and vcpu_block's add_wait_queue() are
> ignored, possibly resulting in hangs.
>
> Also make sure that atomic_inc and waitqueue_active tests happen in the
> specified order, otherwise the following race is open:
>
> CPU0 CPU1
> if (waitqueue_active(wq))
> add_wait_queue()
> if (!atomic_read(pit_timer->pending))
> schedule()
> atomic_inc(pit_timer->pending)
>
> Which is not an issue for the APIC timer due to migration logic.
>
>

Nasty. I hope we can get Dor's interrupt injection notification
working, so we don't have to handle these bugs.

> Index: kvm/virt/kvm/kvm_main.c
> ===================================================================
> --- kvm.orig/virt/kvm/kvm_main.c
> +++ kvm/virt/kvm/kvm_main.c
> @@ -752,6 +752,7 @@ void mark_page_dirty(struct kvm *kvm, gf
> }
> }
>
> +#ifdef CONFIG_X86
> /*
> * The vCPU has executed a HLT instruction with in-kernel mode enabled.
> */
> @@ -765,6 +766,7 @@ void kvm_vcpu_block(struct kvm_vcpu *vcp
>

This breaks ia64 (and shouldn't s390 use this too?)
> * We will block until either an interrupt or a signal wakes us up
> */
> while (!kvm_cpu_has_interrupt(vcpu)
> + && !kvm_cpu_has_pending_timer(vcpu)
>

I guess the fix is to stub this out for the other archs.

--
Any sufficiently difficult bug is indistinguishable from a feature.