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

[Gleb Natapov]

On Tue, Nov 26, 2013 at 01:47:03PM +0100, Paolo Bonzini wrote:
> 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?
> 
It should be rate limited somehow. Since it guest triggarable guest may cause
host to allocate a lot of memory this way.

Is this about MSI interrupt affinity? IIRC changing INT interrupt
affinity should not trigger kvm_set_irq_routing update. If this is about
MSI only then what about changing userspace to use KVM_SIGNAL_MSI for
MSI injection?

--
                        Gleb.