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Re: [Qemu-devel] [RFC] create a single workqueue for each vm to update v
From: |
Zhanghaoyu (A) |
Subject: |
Re: [Qemu-devel] [RFC] create a single workqueue for each vm to update vm irq routing table |
Date: |
Thu, 28 Nov 2013 06:27:05 +0000 |
>>>>> > >>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 They may still use outdated routing table while a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.
>
If we use call_rcu()(Not consider the problem that Gleb pointed out
temporarily) instead of synchronize_rcu(), should we still ensure this?
Thanks,
Zhang Haoyu
>If we want to ensure, we need to use a different mechanism for synchronization
>than the global RCU. QRCU would work; readers are not wait-free but only if
>there is a concurrent synchronize_qrcu, which should be rare.
>
>Paolo
- Re: [Qemu-devel] [RFC] create a single workqueue for each vm to update vm irq routing table, (continued)
- Re: [Qemu-devel] [RFC] create a single workqueue for each vm to update vm irq routing table, Michael S. Tsirkin, 2013/11/26
- Re: [Qemu-devel] [RFC] create a single workqueue for each vm to update vm irq routing table, Gleb Natapov, 2013/11/26
- Re: [Qemu-devel] [RFC] create a single workqueue for each vm to update vm irq routing table, Avi Kivity, 2013/11/26
- Re: [Qemu-devel] [RFC] create a single workqueue for each vm to update vm irq routing table, Gleb Natapov, 2013/11/26
- Re: [Qemu-devel] [RFC] create a single workqueue for each vm to update vm irq routing table, Paolo Bonzini, 2013/11/26
- Re: [Qemu-devel] [RFC] create a single workqueue for each vm to update vm irq routing table, Avi Kivity, 2013/11/26
- Re: [Qemu-devel] [RFC] create a single workqueue for each vm to update vm irq routing table, Gleb Natapov, 2013/11/26
- Re: [Qemu-devel] [RFC] create a single workqueue for each vm to update vm irq routing table,
Zhanghaoyu (A) <=
- Re: [Qemu-devel] [RFC] create a single workqueue for each vm to update vm irq routing table, Paolo Bonzini, 2013/11/28
- Re: [Qemu-devel] [RFC] create a single workqueue for each vm to update vm irq routing table, Gleb Natapov, 2013/11/28
- Re: [Qemu-devel] [RFC] create a single workqueue for each vm to update vm irq routing table, Paolo Bonzini, 2013/11/28
- Re: [Qemu-devel] [RFC] create a single workqueue for each vm to update vm irq routing table, Gleb Natapov, 2013/11/28
- Re: [Qemu-devel] [RFC] create a single workqueue for each vm to update vm irq routing table, Avi Kivity, 2013/11/28
- Re: [Qemu-devel] [RFC] create a single workqueue for each vm to update vm irq routing table, Paolo Bonzini, 2013/11/28
- Re: [Qemu-devel] [RFC] create a single workqueue for each vm to update vm irq routing table, Avi Kivity, 2013/11/28
- Re: [Qemu-devel] [RFC] create a single workqueue for each vm to update vm irq routing table, Paolo Bonzini, 2013/11/28
- Re: [Qemu-devel] [RFC] create a single workqueue for each vm to update vm irq routing table, Avi Kivity, 2013/11/28
- Re: [Qemu-devel] [RFC] create a single workqueue for each vm to update vm irq routing table, Gleb Natapov, 2013/11/28