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

[Paul Brook <paul@codesourcery.com>]

On Thursday 21 May 2009, Avi Kivity wrote:
> 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.

So presumably if a device triggers an APIC interrupt using a write that isn't 
one of the currently configured PCI devices, it all explodes horribly?

Paul