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

[Markus Armbruster]

Marcel Apfelbaum <address@hidden> writes:

> 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.

I have no opinion on two devices vs. one device + property in this
particular case, I just want to interject that I'd expect the difference
in maintaince to be negligible.

A second device basically takes a copy of the TypeInfo with some
(trivial) init function to make it different.  Might be a few more lines
of code than adding a property, but in complexity, it's a wash.

In case you plan to get rid of the old variant: with two devices, you
deprecate and later delete the old device.  With device + property, you
deprecate setting the property, and later delete it.  The former might
be a bit easier to document.

>>> I wanted to get rid of the internal pci-bridge as a default, and this
>>> is why pxb and pxb-pcie are he same device now (except bus type)