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

[Paolo Bonzini]

On 16/11/2015 10:52, Marcel Apfelbaum wrote:
> On 11/16/2015 10:40 AM, Paolo Bonzini wrote:
>>
>>
>> On 15/11/2015 16:39, Marcel Apfelbaum wrote:
>>>   Addressed Gerd Hoffmann comments:
>>>   - Added x-enable-internal-bridge compat property to keep the PCI
>>>     bridge for older machine to avoid breaking migration.
>>
>> This will break PXB command lines written for "-M pc" in QEMU <= 2.5.
>> Technically there's no prohibition against doing so, but we've never
>> done it; I'm not sure why we should start doing so now.
> 
> Hi Paolo,
> 
> Can you please explain how does it break the command line?
> 
> For QEMU < 2.5 we automatically have the  x-enable-internal-bridge
> compat property "on"
> so the command line remains the same.
> 
> For QEMU >= 2.5 is "off" by default and the bridge disappear, without
> command line changes.

If you were using "-M pc", and you expected "-device" to place things
under the PCI-to-PCI bridge, that command line will not work anymore I
think.

Management tools can work around it.  They can always set the new
property to off---which by the way means it should not be "x"
prefixed---and assume the PCI-to-PCI bridge is there if the property
doesn't exist.  But it's inconvenient, and we try to avoid lockstep
changes to QEMU and management.

Paolo

>> Can we have two devices, on for a PCI expander bridge and one for a PCIe
>> expander bridge?
> 
> Gerd also asked for this, OK, I am going for it.
> 
> Thanks,
> Marcel
> 
>>
>> Paolo
>>
>