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


From: Marcel Apfelbaum
Subject: Re: [Qemu-devel] [PATCH V2] pci: fixes to allow booting from extra root pci buses.
Date: Fri, 12 Jun 2015 15:17:27 +0300
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.5.0

On 06/12/2015 09:00 AM, Gerd Hoffmann wrote:
   Hi,

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.

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? 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.

Thanks,
Marcel



cheers,
   Gerd






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