On Fri, 13 Jan 2017 09:57:36 +1100 David Gibson <david@xxxxxxxxxxxxxxxxxxxxx> wrote: > 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. > Well, there might be someone now :) Michael Roth had posted a RFC patchset back in 2015: https://lists.gnu.org/archive/html/qemu-ppc/2015-04/msg00275.html I'll start from here. Cheers. -- Greg
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