[libvirt] [PATCH v3 19/18] [RFC] qemu: assure there are always at least 4 open pcie-root-ports for hotplug

[Andrea Bolognani]

On Fri, 2016-10-14 at 16:54 +0200, Ján Tomko wrote:
> Also, would it be possible to make this feature of leaving
> free hot-pluggable slots opt-in?
> 
> E.g. a <controller model='pcie-root-port'> without an index
> would be considered a port where we should not put devices
> with auto-assigned addresses.
> 
> (This might actually be more ugly than the proposed solution,
> or the 'freeHotplugSlots' attribute, but I dislike implicit
> device additions after having model='none' memballoon and usb
> controller)

If we can find a way to make it opt-in without it turning
out incredibly ugly (eg. 'freeHotplugSlots' or similar
paramers) or requiring too much knowledge from the users /
management applications (eg. "you have x legacy PCI devices
and y PCI Express devices, that means adding x + y + z + 3
pcie-root-ports before passing the XML to libvirt") I'm all
for that. So far we haven't been able to come up with
anything like that, though :)

I think the current proposal is fairly okay because it allows
for easy opt-out, and if we reduce the number of extra ports
to just one as per your comments I don't think people will
care at all. As far as I'm concerned, that's a very good
balance; it's certainly better than the status quo of adding
a completely useless dmi-to-pci-bridge + pci-bridge
combination to every single q35 guest.

-- 
Andrea Bolognani / Red Hat / Virtualization