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

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 :)