On Fri, 13 Jan 2017 09:57:36 +1100 David Gibson <david@gibson.dropbear.id.au> 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