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

[Kevin O'Connor]

On Fri, Jun 12, 2015 at 03:17:27PM +0300, Marcel Apfelbaum wrote:
> On 06/12/2015 09:00 AM, Gerd Hoffmann wrote:
> >>On each boot, coreboot might decide to assign a different bus id to
> >>the extra roots (for example, if a device with a PCI bridge is
> >>inserted and it's bus allocation causes bus ids to shift).
> >>Technically, coreboot could even change the order extra buses are
> >>assigned bus ids, but doesn't today.
> >>
> >>This was seen on several AMD systems - I'm told at least some Intel
> >>systems have multiple root buses, but the bus numbers are just hard
> >>wired.
> >
> >This is how the qemu pxb works: root bus numbers are a config option for
> >the root bridge device, i.e. from the guest point of view they are
> >hard-wired.
> Exactly. In our case, the HW assigns the PXB bus bumber, and again,
> I saw this also on real HW with multiple buses, the bus nr comes
> from ACPI, meaning the vendor.

I'm confused where ACPI comes into this.  In all cases I know of, the
firmware generates the ACPI tables to match the hardware.  I've never
heard of hardware configuring itself from the ACPI tables.

> Let's focus on the problem in hand: We need a way for QEMU to write
> some fw path on bootorder fw_config file and both Seabios/OVMF need
> to know how to correctly map this to the actual device.
> 
> If the boot device is behind a PXI extra root bus, there is a need
> not only to differentiate the root bus but also to know *which
> one*. So we need the bus number, what other way is there?

The submitted patch changed the mechanism already in SeaBIOS.  I'm not
claiming the existing mechanism was perfect, but lets not claim that
it's not possible either.

>As Gerd
> mentioned, the PXB bus number is provided in QEMU command line,
> meaning hard-wired.
> 
> We can of course, as Laszlo suggested, add an extra condition the
> use of this path: /address@hidden/ on running in QEMU in order not
> to interfere with other HW. Less pretty but more robust.

-Kevin