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

[Michael S. Tsirkin]

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.


> 
> >Paolo
> >
> >>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)