Re: [Qemu-devel] [PATCH V2 0/4] hw/pcie: Multi-root support for Q35

[Laine Stump <laine@redhat.com>]

On 11/16/2015 05:37 AM, Michael S. Tsirkin wrote:
> On Mon, Nov 16, 2015 at 12:34:11PM +0200, Marcel Apfelbaum wrote:
>> On 11/16/2015 12:11 PM, Paolo Bonzini wrote:
>>>
>>>
>>> On 16/11/2015 11:10, Marcel Apfelbaum wrote:
>>>>> What would you lose?  Hotplug?
>>>>
>>>> Without the bridge? Yes. However the user can add it manually the
>>>> pci-bridge and have it anyway.
>>>
>>> Ok, I guess that's more or less acceptable.  It's still ugly however, to
>>> the point that I wonder if we should rename the device and call the old
>>> one a failed experiment.
>>>
>>
>> I guess we can rename the pxb to extra-root or something, but in this way
>> will have a deprecated/duplicated device to support and kill in the future.
>>
>> Why not use the compat property as it is?
>> Again, the command line *remains* the same, the difference is where the
>> devices associated with the pxb will land: on the secondary bus (for QEMU < 2.5)
>> or on the root bus itself (QEMU >= 2.5).
>>
>> I know is guest visible, but the guest will see one of them depending on the machine type.
>>
>> Regarding the splitting of pxb into 2 devices (pci/pcie), I have nothing against it,
>> but because the implementation is *exactly* the same I think we should gain more
>> by maintaining one device.
>>
>>
>> Thanks,
>> Marcel
>
> Yes, I think you want a new "pci-extender" device which is just the extender.
> Then existing pxb will create both it and the bridge behind it.
> Maybe creating pxb which is extender+bridge was a mistake, I don't know,
> but we shipped it in QEMU so we support it.

At least from libvirt's POV, pxb support hasn't yet been added, so even 
though qemu has shipped it, libvirt (and thus ovirt and openstack at 
least) hasn't used it, and from the sounds of this discussion, it 
probably won't; we'll just directly support the new model for 440fx and q35.