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

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On 11/26/2013 06:24 PM, Gleb Natapov wrote:
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 They may still use outdated routing table while a vcpu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


Suppose the guest did not disable_irq() and enable_irq(), but instead had a pci read where you have the enable_irq(). After the read you cannot have a stale irq (assuming the read flushes the irq all the way to the APIC).

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