Re: [Qemu-devel] [QEMU][RFC PATCH 4/6] xen-pci: Register PCI in Xen

[Stefano Stabellini <stefano.stabellini@eu.citrix.com>]

On Mon, 26 Mar 2012, Avi Kivity wrote:
> > > What about if two BARs (from different devices) are configured for the
> > > same address ranges?
> >
> > I think that it should have the same chance of happening as if there was
> > just one QEMU, because from the guest OS and firmware POV the emulated
> > hardware is the same. In any case Xen would return an error and QEMU can
> > either exit or try to cope with it.
> 
> How can qemu cope?  In a normal situation it's aware of all the devices,
> here it's not aware of any device (except the one it is managing).
> 
> You're trying to convert a hierarchical problem into a flat one with no
> communication.  What happens if one of the devices is a PCI-PCI bridge
> and it turns off its PCI window?  The devices behind it should no longer
> respond, yet they know nothing about it.
> 
> I think you need to preserve the hierarchy.  The host-pci bridge needs
> to talk to devices behind it, (as does a pci-pci bridge).

PCI bridges are a problem, in fact I was thinking to just assume that
there are no PCI Bridges with PCI devices behind them other than the
Host PCI Bridge for now.

There is going to be a need for some kind of communication between the
multiple QEMU instances, for example for ACPI power state changes and
PCI Bridges, like you wrote. So I think that at some point we'll need to
introduce a type of message that originates from one of the QEMU
instances (the first one especially) and goes to all the others.
It should probably be relayed by the hypervisor.