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Date: Tue, 26 Nov 2013 13:47:03 +0100
From: Paolo Bonzini <pbonzini@redhat.com>
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Subject: Re: [Qemu-devel] [RFC] create a single workqueue for each vm to
 update vm irq routing table
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To: "Zhanghaoyu (A)" <haoyu.zhang@huawei.com>
Cc: "Huangweidong (C)" <weidong.huang@huawei.com>, KVM <kvm@vger.kernel.org>, Gleb Natapov <gleb@redhat.com>, "Michael S. Tsirkin" <mst@redhat.com>, "Jinxin (F)" <jinxin712@huawei.com>, Luonengjun <luonengjun@huawei.com>, "qemu-devel@nongnu.org" <qemu-devel@nongnu.org>, Zanghongyong <zanghongyong@huawei.com>

Il 26/11/2013 13:40, Zhanghaoyu (A) ha scritto:
> When guest set irq smp_affinity, VMEXIT occurs, then the vcpu thread will IOCTL return to QEMU from hypervisor, then vcpu thread ask the hypervisor to update the irq routing table,
> in kvm_set_irq_routing, synchronize_rcu is called, current vcpu thread is blocked for so much time to wait RCU grace period, and during this period, this vcpu cannot provide service to VM,
> so those interrupts delivered to this vcpu cannot be handled in time, and the apps running on this vcpu cannot be serviced too.
> It's unacceptable in some real-time scenario, e.g. telecom. 
> 
> So, I want to create a single workqueue for each VM, to asynchronously performing the RCU synchronization for irq routing table, 
> and let the vcpu thread return and VMENTRY to service VM immediately, no more need to blocked to wait RCU grace period.
> And, I have implemented a raw patch, took a test in our telecom environment, above problem disappeared.

I don't think a workqueue is even needed.  You just need to use call_rcu
to free "old" after releasing kvm->irq_lock.

What do you think?

Paolo