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

[Paolo Bonzini]

Il 26/11/2013 14:18, Avi Kivity ha scritto:
> 
>> I don't think a workqueue is even needed.  You just need to use call_rcu
>> to free "old" after releasing kvm->irq_lock.
>>
>> What do you think?
> 
> Can this cause an interrupt to be delivered to the wrong (old) vcpu?

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.

There is still the problem that Gleb pointed out, though.

Paolo

> The way Linux sets interrupt affinity, it cannot, since changing the
> affinity is (IIRC) done in the interrupt handler, so the next interrupt
> cannot be in flight and thus pick up the old interrupt routing table.
> 
> However it may be vulnerable in other ways.
> 
>