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

[Klaus Jensen]

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.

> 2. How can I debug this kind of cross QEMU-KVM problems?

Not sure how to directly "debug" it, but there is `perf kvm` to get
information about what is happing in the kvm subsystem.

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