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Re: [libvirt] new libvirt "pci" controller type and pcie/q35 (was Re: [PATCH 4/7] add pci-bridge controller type)



On Mon, Apr 08, 2013 at 12:37:49PM -0400, Laine Stump wrote:
> On 04/05/2013 03:26 PM, Alex Williamson wrote:
> I think we're starting to get closer to the concrete problem that's
> bothering me. As I understand it (and again - "what I understand" has
> repeatedly been shown to be incorrect in this thread :-):
> 
> * Ihere are multiple different types of devices that provide a bus with
> 1 or more "slots" that PCI devices (e.g., the virtio-net-pci device, the
> e1000 network device, etc) can be plugged into.
> 
> * In the config for those devices, there is a required (auto-generated
> if not explicitly provided) <address> element that indicates what
> controller that device is plugged into e.g.:
> 
>     <interface type='direct'>
>       ...
>       <address type='pci' domain='0' bus='0' slot='3' function='0'/>
>       ...
>     </interface>
> 
> * domain is always hardcoded to 0, and in the past bus was also always
> hardcoded to 0 because until now there has only been a single place
> where PCI devices could be connected - the builtin pci.0 bus, which is a
> part of the basic "pc" (and some others) virtual machine and provides 32
> slots.
> 
> * Now we are adding the ability to define new PCI buses, for now just a
> single kind - a pci-bridge controller, which itself must connect to an
> existing PCI slot, and provides 32 new PCI slots. But in the future
> there will be more different types of controllers that provide one or
> more PCI slots where PCI devices/controllers can be plugged in.
> 
> * In these patches adding support for pci-bridge, we are making the
> assumption that there is a 1:1 correspondence between the "index='n'"
> attribute of the pci-bridge controller and the "bus='n'" attribute of
> the <address> element in devices that will be plugged into that
> controller. So for example if we have:
> 
> 
>    <controller type='pci-bridge' index='1'>
>       <address type='pci' domain='0' bus='0' slot='10' function='0'/>
>    </controller>
> 
> and then change the <interface> definition above to say "bus='1'", that
> interface device will plug into this new bus at slot 3.
> 
> * So let's assume that we add a new controller called "dmi-to-pci-bridge":
> 
>   <controller type='dmi-to-pci-bridge' index='0'/>
> 
> Ignoring for now the question of what address we give in the definition
> of *this* device (which is itself problematic - do we need a new "pcie"
> address type?), if some device is then defined with
> 
> 
>    <address type='pci bus='0' .../>
> 
> How do we differentiate between that meaning "the pci-ptp controller
> that is index='0'" and "the pci-bridge controller that is index='0'"? Do
> we need to expand our <address> element further? If, as I think you
> suggest, we have multiple different kinds of controllers that provide
> PCI slots, each with its own namespace, the current pci address element
> is inadequate to unambiguously describe where a pci device should be
> plugged in.

Hmm yes, you're right - as long as we only have   <adress type='pci'>
then all <controller> elements should use  type='pci' too, and we should
just distinguish based on the model name of the controller. So ignore
my previous suggestion to have 'pci-bridge' and 'pci-root' types, we
can only use  type='pci' on <controller> elements.

Daniel
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