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

[Marcel Apfelbaum]

On 11/16/2015 11:56 AM, Paolo Bonzini wrote:
> 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.

Actually the command line would work. The devices will be placed on the root bus
instead of the pci bridge bus. The trick here is that we name the bus the 
devices
will be attached to by the pxb id.
 - When we have the bridge -> the pxb id will be used for the secondary bus
 - when don't have the bridge -> the pxb id wil be used for the root bus.
Both times the command line remains -device pxb,id=my_root -device 
e1000,bus=my_root


> 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.

I understand, I'll implement this as a new device "pxb-pcie"

Thanks,
Marcel

> 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