Re: [Qemu-devel] [PATCH] RFC kvm irqfd: add directly mapped MSI IRQ support

[Alex Williamson]

On Fri, 2013-06-21 at 15:12 +1000, Benjamin Herrenschmidt wrote:
> On Thu, 2013-06-20 at 22:46 -0600, Alex Williamson wrote:
> > Maybe you could add a device parameter to kvm_irqchip_add_msi_route so
> > that it can be implemented on POWER without this pci_bus_map_msi
> > interface that seems very unique to POWER.  Thanks,
> 
> You mean unique to all non-x86 ? :-)
> 
> I believe almost everybody eventually turn MSIs into "normal"
> interrupts...
> 
> Most often than not, the logic to do so is in the PCI Host Bridge.
> 
> The whole concept of passing the message address/data accross the
> user/kernel interface is an x86 crackpotery but as is the entire
> remapping/routing layer so ... :-)

Regardless, this is exactly what kvm_irqchip_add_msi_route does.  It
says, here's an MSIMessage, give me an IRQ that sends that.  In the x86
case, that means pick a free IRQ and program it to send that MSIMessage
when we hit the irqfd.  In the case of POWER it means lookup which IRQ
gets fired by that MSIMessage and return it.  In a non-accelerated QEMU
case I'd think msi_notify() would write the MSIMessage to this IRQ
remapper device and let it toggle the next qemu_irq down the line.  If
we ever add an IOMMU based IRQ remapper to the x86 model, we'd need
something similar.  Thanks,

Alex