On 01/12/2017 05:31 AM, 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.
And of course the reason we don't want to add "too many" extra controllers by default is so that we don't end up with *all* guests burdened with extra hardware they don't need or want. The libguestfs appliance is one example of a libvirt consumer that definitely doesn't want extra baggage in its guests - guest startup time is very important to libguestfs, so any addition to the hardware list is looked upon with disappointment.
Of course this all will be moot once we can hotplug PHBs :)
Will the guest OSes handle that properly? I remember being told that Linux, for example, doesn't scan the new bus for devices when a new controller is added, making it pointless to hotplug a PCI controller (as usual, it could be that I'm remembering incorrectly...)