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

On Thu, Jan 12, 2017 at 11:31:35AM +0100, Andrea Bolognani wrote: > 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. Hm, ok. Well I guess the limitation is the same as on x86, so it shouldn't surprise people. > Of course this all will be moot once we can hotplug PHBs :) Yes. Unfortunately, nobody's actually working on that at present.