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 They may still use outdated routing table while a vcpu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().