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

[樊金昊]

> The driver will create the cq with an allocated vector, but it's not 
activated
> until after the driver wires it up to a handler. I think that's what you're
> observing with the incomplete MSIx table entry on creation.

Agreed. I digged through pci_request_irq()'s call chain and found 
pci_write_msi_msg() was called in the end.

Now to implement irqfd support, we need to register the (complete) MSI message 
in KVM so that KVM can directly send the interrupt when we signal the irqfd.
My prior implementation delayed each CQ's MSI message registration to its first
nvme_post_cqes(). I'm not sure whether this is a good choice. What do you think
about this approach? 

BTW, since we skip QEMU's MSI-x emulation with irqfd, we need to record the
mask status of each interrupt vector. QEMU provides msix_set_vector_notifiers()
to help us call handlers on each mask and unmask event. But this function works
on a per-device basis. I guess it is best to call msix_set_vector_notifiers()
after all CQs are created. But I think qemu-nvme can't tell when the host has
finished CQ creation. Where do you think is the best place we register the
mask/unmask callbacks? Is it OK to put it at, say, the first nvme_post_cqes()
of the whole device?

Thanks,
Jinhao Fan