Re: [Qemu-devel] [RFC] create a single workqueue for each vm to update vm irq routing table

Paolo Bonzini
2013-11-26T14:47:10Z

Il 26/11/2013 15:36, Avi Kivity ha scritto:
>
> No, this would be exactly the same code that is running now:
>
> mutex_lock(&kvm->irq_lock);
> old = kvm->irq_routing;
> kvm_irq_routing_update(kvm, new);
> mutex_unlock(&kvm->irq_lock);
>
> synchronize_rcu();
> kfree(old);
> return 0;
>
> Except that the kfree would run in the call_rcu kernel thread instead of
> the vcpu thread. But the vcpus already see the new routing table after
> the rcu_assign_pointer that is in kvm_irq_routing_update.
>
> I understood the proposal was also to eliminate the synchronize_rcu(),
> so while new interrupts would see the new routing table, interrupts
> already in flight could pick up the old one.

Isn't that always the case with RCU? (See my answer above: "the vcpus
already see the new routing table after the rcu_assign_pointer that is
in kvm_irq_routing_update").

If you eliminate the synchronize_rcu, new interrupts would see the new
routing table, while interrupts already in flight will get a dangling
pointer.

Paolo