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

Gleb Natapov
2013-11-26T16:24:21Z

On Tue, Nov 26, 2013 at 04:20:27PM +0100, Paolo Bonzini wrote:
> Il 26/11/2013 16:03, Gleb Natapov ha scritto:
> >>>> > >>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 They may still use outdated routing table while a vcpu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().
>
> Avi's point is that, after the VCPU resumes execution, you know that no
> interrupt will be sent to the old destination because
> kvm_set_msi_inatomic (and ultimately kvm_irq_delivery_to_apic_fast) is
> also called within the RCU read-side critical section.
>
> Without synchronize_rcu you could have
>
> VCPU writes to routing table
> e = entry from IRQ routing table
> kvm_irq_routing_update(kvm, new);
> VCPU resumes execution
> kvm_set_msi_irq(e, &irq);
> kvm_irq_delivery_to_apic_fast();
>
> where the entry is stale but the VCPU has already resumed execution.
>
So how is it different from what we have now:

disable_irq()
VCPU writes to routing table
e = entry from IRQ routing table
kvm_set_msi_irq(e, &irq);
kvm_irq_delivery_to_apic_fast();
kvm_irq_routing_update(kvm, new);
synchronize_rcu()
VCPU resumes execution
enable_irq()
receive stale irq

--
Gleb.