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

[Gleb Natapov <gleb@redhat.com>]

On Sat, Nov 20, 2010 at 10:17:09PM +0200, Michael S. Tsirkin wrote:
> 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
Most familiar to whom? It looks like you identify yourself with most of
qemu users, but if most qemu users are like you then qemu has not enough
users :) Most users that consider themselves to be "advanced" may know
what eth1 or /dev/sdb means. This doesn't mean we should provide
"device_del eth1" or "device_add /dev/sdb" command though. 

More important is that "domain" (encoded as number like you used to)
and "bus number" has no meaning from inside qemu. So while I said many
times I don't care about exact CLI syntax to much it should make sense
at least. It can use id to specify PCI bus in CLI like this:
device_del pci.0:1.1. Or it can even use device id too like this:
device_del pci.0:ide.0. Or it can use HW topology like in FO device
path. But doing ah-hoc device enumeration inside qemu and then using it
for CLI is not it.

> 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.
It looks like you confused ACPI _SEG for something it isn't. ACPI spec
says that PCI segment group is purely software concept managed by system
firmware. In fact one segment may include multiple PCI host bridges. _SEG
is not what OSPM uses to tie HW resource to ACPI resource. It used _CRS
(Current Resource Settings) for that just like OF. No surprise there.

> 
> 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.
> 
Describing HW topology is the only way to unambiguously describe device to
something or someone outside qemu and have persistent device naming
between different HW configuration.

--
			Gleb.