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

["Michael S. Tsirkin" <mst@redhat.com>]

On Thu, May 21, 2009 at 03:50:18PM +0100, Paul Brook wrote:
> > >>> kvm has no business messing with the PCI device code.
> > >>
> > >> kvm has a fast path for irq injection.  If qemu wants to support it we
> > >> need some abstraction here.
> > >
> > > Fast path from where to where? Having the PCI layer bypass/re-implement
> > > the APIC and inject the interrupt directly into the cpu core sounds a
> > > particularly bad idea.
> >
> > 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.
> 
> Paul

As far as I can tell, at least on Intel, MSI interrupts are not MMIO writes.
They are PCI memory writes to a hard-coded address range that
are passed to APIC. I don't think MMIO writes can triger MSI,
or at least this does not seem to be documented.

-- 
MST