Re: [Qemu-devel] [PATCH V2] pci: fixes to allow booting from extra root pci buses.

[Michael S. Tsirkin]

On Mon, Jun 15, 2015 at 12:50:08PM +1000, Benjamin Herrenschmidt wrote:
> On Sun, 2015-06-14 at 17:59 -0400, Kevin O'Connor wrote:
> > There are x86 systems with multiple separate PCI root buses where one
> > can access the pci config space of all the buses using the same 0x0cf8
> > IO space.  During system setup, the multiple PCI root buses are each
> > configured to only respond to PCI config accesses within its range of
> > bus numbers.  So if "root1" is configured for bus ids between 64-128,
> > then it will only forward the request if the bus id in the request is
> > between 64-128.
> > 
> > I suspect in your PPC example that the separate root buses all had
> > separate io/memory space as well and thus were completely separate.
> > (That is, they don't share the equivalent of IO 0x0cf8.)  If so,
> > that's different from how the x86 qemu code and the x86 systems I was
> > discussing above work.
> 
> Correct, my point is that qemu shouldn't be made to rely on the stable
> bus numbers. Why not use the mmconfig address instead ?

These are traditional pci buses, so they don't have an mmconfig.

> That way you can
> factor the bus number in via an offset if it's relevant or provide a
> completely different address if the busses are separate.
> 
> Ben.
>