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

["Michael S. Tsirkin" <mst@redhat.com>]

On Fri, Nov 19, 2010 at 10:38:42PM +0200, Gleb Natapov wrote:
> On Fri, Nov 19, 2010 at 06:02:58PM +0100, Markus Armbruster wrote:
> > "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
> > 
> FWIW openbios allows this kind of abbreviation.
> 
> > 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.
> 
> --
> 			Gleb.


I think that the domain (PCI segment group), bus, slot, function way to
address pci devices is still the most familiar and the easiest to map to
functionality in the guests.  Qemu is buggy in the moment in that is
uses the bus addresses assigned by guest and not the ones in ACPI,
but that can be fixed.

That should be enough for e.g. device_del. We do have the need to
describe the topology when we interface with firmware, e.g. to describe
the ACPI tables themselves to qemu (this is what Gleb's patches deal
with), but that's probably the only case.

-- 
MST