Re: [RFC] hw/nvme: Use irqfd to send interrupts

[Klaus Jensen <its@irrelevant.dk>]

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On Jul 12 14:26, Klaus Jensen wrote:
> On Jul  9 12:35, Jinhao Fan wrote:
> > Use irqfd to directly notify KVM to inject interrupts. This is done by
> > registering a virtual IRQ(virq) in KVM and associate the virq with an
> > irqfd, so that KVM can directly inject the interrupt when it receives
> > notification from the irqfd. This approach is supposed to improve 
> > performance because it bypasses QEMU's MSI interrupt emulation logic.
> > 
> > However, I did not see an obvious improvement of the emulation KIOPS:
> > 
> > QD      1   4  16  64 
> > QEMU   38 123 210 329
> > irqfd  40 129 219 328
> > 
> > I found this problem quite hard to diagnose since irqfd's workflow
> > involves both QEMU and the in-kernel KVM. 
> > 
> > Could you help me figure out the following questions:
> > 
> > 1. How much performance improvement can I expect from using irqfd?
> 
> This is a level of QEMU/KVM that I am by no means an expert on and I
> would have to let the broader QEMU community comment on this.
> 

In any case, I'm wary about adding this level of kvm-dependence in the
device. This wont work on non-kvm platforms any more.

I think you should put irqfd on hold and focus on iothreads :)

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