[libvirt] Proposal PCI/PCIe device placement on PAPR guests

[Andrea Bolognani]

On Mon, 2017-01-09 at 10:46 +1100, David Gibson wrote:
> > >    * To allow for hotplugged devices, libvirt should also add a number
> > >      of additional, empty vPHBs (the PAPR spec allows for hotplug of
> > >      PHBs, but this is not yet implemented in qemu).
> > 
> > "A number" here will have to mean "one", same number of
> > empty PCIe Root Ports libvirt will add to a newly-defined
> > q35 guest.
> 
> Umm.. why?

Because some applications using libvirt would inevitably
start relying on the fact that such spare PHBs are
available, locking us into providing at least the same
number forever. In other words, increasing the amount at
a later time is always possible, but decreasing it isn't.
We did the same when we started automatically adding PCIe
Root Ports to q35 machines.

The rationale is that having a single spare hotpluggable
slot is extremely convenient for basic usage, eg. a simple
guest created by someone who's not necessarily very
familiar with virtualization; on the other hand, if you
are actually deploying in production you ought to conduct
proper capacity planning and figure out in advance how
many devices you're likely to need to hotplug throughout
the guest's life.

Of course this all will be moot once we can hotplug PHBs :)

-- 
Andrea Bolognani / Red Hat / Virtualization