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

[Paolo Bonzini]

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