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

[Paul Brook <paul@codesourcery.com>]

> >> The PCI bus doesn't need any special support (I think) but something on
> >> the other end needs to interpret those writes.
> >
> > Sure. But there's definitely nothing PCI specific about it. I assumed
> > this would all be contained within the APIC.
>
> MSIs are defined by PCI and their configuration is done using the PCI
> configuration space.

A MSI is just a regular memory write, and the PCI spec explicitly states that 
a target (e.g. the APIC) is unable to distinguish between a MSI and any other 
write. The PCI config bits just provide a way of telling the device where/what 
to write.

> >> In any case we need some internal API for this, and qemu_irq looks like
> >> a good choice.
> >
> > What do you expect to be using this API?
>
> virtio, emulated devices capable of supporting MSI (e1000?), device
> assignment (not yet in qemu.git).

It probably makes sense to have common infrastructure in pci.c to 
expose/implement device side MSI functionality. However I see no need for a 
direct API between the device and the APIC. We already have an API for memory 
accesses and MMIO regions. I'm pretty sure a system could implement MSI by 
pointing the device at system ram, and having the CPU periodically poll that.

Paul