Re: [Qemu-devel] Re: [PATCH] PCI: Bus number from the bridge, not the device

[Markus Armbruster <armbru@redhat.com>]

"Michael S. Tsirkin" <mst@redhat.com> writes:

> On Tue, Nov 09, 2010 at 11:41:43AM +0900, Isaku Yamahata wrote:
>> On Mon, Nov 08, 2010 at 06:26:33PM +0200, Michael S. Tsirkin wrote:
>> > Replace bus number with slot numbers of parent bridges up to the root.
>> > This works for root bridge in a compatible way because bus number there
>> > is hard-coded to 0.
>> > IMO nested bridges are broken anyway, no way to be compatible there.
>> > 
>> > 
>> > Gleb, Markus, I think the following should be sufficient for PCI.  What
>> > do you think?  Also - do we need to update QMP/monitor to teach them to
>> > work with these paths?
>> > 
>> > This is on top of Alex's patch, completely untested.
>> > 
>> > 
>> > pci: fix device path for devices behind nested bridges
>> > 
>> > We were using bus number in the device path, which is clearly
>> > broken as this number is guest-assigned for all devices
>> > except the root.
>> > 
>> > Fix by using hierarchical list of slots, walking the path
>> > from root down to device, instead. Add :00 as bus number
>> > so that if there are no nested bridges, this is compatible
>> > with what we have now.
>> 
>> This format, Domain:00:Slot:Slot....:Slot.Function, doesn't work
>> because pci-to-pci bridge is pci function.
>> So the format should be
>> Domain:00:Slot.Function:Slot.Function....:Slot.Function
>> 
>> thanks,
>
> Hmm, interesting. If we do this we aren't backwards compatible
> though, so maybe we could try using openfirmware paths, just as well.

Whatever we do, we need to make it work for all (qdevified) devices and
buses.

It should also be possible to use canonical addressing with device_add &
friends.  I.e. permit naming a device by (a unique abbreviation of) its
canonical address in addition to naming it by its user-defined ID.  For
instance, something like

   device_del /pci/@1,1

in addition to

   device_del ID

Open Firmware is a useful source of inspiration there, but should it
come into conflict with usability, we should let usability win.