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

[Laine Stump]

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...)