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

[Zhanghaoyu (A)]

Hi all,

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.

Any better ideas?

Thanks,
Zhang Haoyu