Re: [Qemu-devel] [PATCH 6/8] pci: Simpler implementation of primary PCI bus

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

On Wed, May 29, 2013 at 08:06:42PM +1000, David Gibson wrote:
> On Wed, May 29, 2013 at 12:55:53PM +0300, Michael S. Tsirkin wrote:
> > On Wed, May 29, 2013 at 07:43:41PM +1000, David Gibson wrote:
> > > On Thu, May 23, 2013 at 10:16:27PM +1000, David Gibson wrote:
> > > > On Thu, May 23, 2013 at 02:22:30PM +0300, Michael S. Tsirkin wrote:
> > > > > On Thu, May 09, 2013 at 10:31:10AM +1000, David Gibson wrote:
> > > > > > Currently pci_get_primary_bus() searches the list of root buses for one
> > > > > > with domain 0.  But since host buses are always registered with domain 0,
> > > > > > this just amounts to finding the only PCI host bus.
> > > > > > 
> > > > > > This simplifies the implementation by defining the primary PCI bus to
> > > > > > be the first one registered, using a global variable to track it.
> > > > > > 
> > > > > > Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
> > > > > 
> > > > > Or better: can we just fail if there is more than
> > > > > one root?
> > > > 
> > > > That might work, I'll look into doing that.
> > > 
> > > So, the difficulty with this is that then any machine with multiple
> > > PCI bridges could not use pci_nic_init(), since it calls
> > > pci_get_bus_devfn() which calls pci_find_primary_bus() which would
> > > always fail.  And using pci_nic_init() is more or less mandatory in
> > > the machine_init function to support old-style nic configuration.
> > > 
> > > Suggestions?
> > 
> > You mean multiple PCI roots?
> > Well, there are no legacy machines with multiple roots to support, are
> > there?  So why do we need to support legacy flags for these new
> > configurations?
> 
> Because people expect them.

People can learn, somehow they will learn to add a new root, so they can
learn to use -device too.

So let's make it fail on multiple roots, and output a message along the
lines of "please use -device virtio-net-pci instead".

> Plus on spapr we already support the
> legacy nic options; it would be very strange for them to suddenly
> break when we add a second host bridge.

Not sure who "we" is here. IMHO user should ask for a new
machine type with two roots explicitly.

> -- 
> David Gibson			| I'll have my music baroque, and my code
> david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
> 				| _way_ _around_!
> http://www.ozlabs.org/~dgibson