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

Avi Kivity
2013-11-26T15:24:22Z

On 11/26/2013 05:03 PM, Gleb Natapov wrote:
> On Tue, Nov 26, 2013 at 04:54:44PM +0200, Avi Kivity wrote:
>> On 11/26/2013 04:46 PM, Paolo Bonzini wrote:
>>> 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").
>> With synchronize_rcu(), you have the additional guarantee that any
>> parallel accesses to the old routing table have completed. Since we
>> also trigger the irq from rcu context, you know that after
>> synchronize_rcu() you won't get any interrupts to the old
>> destination (see kvm_set_irq_inatomic()).
> We do not have this guaranty for other vcpus that do not call
> synchronize_rcu(). They may still use outdated routing table while a vcpu
> or iothread that performed table update sits in synchronize_rcu().
>

Consider this guest code:

write msi entry, directing the interrupt away from this vcpu
nop
memset(&idt, 0, sizeof(idt));

Currently, this code will never trigger a triple fault. With the change
to call_rcu(), it may.

Now it may be that the guest does not expect this to work (PCI writes
are posted; and interrupts can be delayed indefinitely by the pci
fabric), but we don't know if there's a path that guarantees the guest
something that we're taking away with this change.