Re: [Qemu-devel] [PATCH] qemu: msi irq allocation api

[Avi Kivity]

Paul Brook wrote:
> > kvm implements the APIC in the host kernel (qemu upstream doesn't
> > support this yet).  The fast path is wired to the in-kernel APIC, not
> > the cpu core directly.
> >
> > The idea is to wire it to UIO for device assignment, to a virtio-device
> > implemented in the kernel, and to qemu.
> >     
>
> I still don't see why you're trying to bypass straight from the pci layer to 
> the apic. Why can't you just pass the apic MMIO writes to the kernel? You've 
> presumably got to update the apic state anyway.
>   

The fast path is an eventfd so that we don't have to teach all the 
clients about the details of MSI.  Userspace programs the MSI details 
into kvm and hands the client an eventfd.  All the client has to do is 
bang on the eventfd for the interrupt to be queued.  The eventfd 
provides event coalescing and is equally useful from the kernel and 
userspace, and can be used with targets other than kvm.

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